Is there or does it just look “cooler”?
More volume for expanding exhaust gases can (if accompanied by other modifications and tuning) yield more horsepower.
Depends on the car, and the design of your current, I assume, stock exhaust. If you think of your engine as a giant air pump (which it kinda is), you have air coming in through the intake, being combusted, then being expelled through the exhaust.
Unless the exhaust is the bottleneck in your system, opening it up through the use more capacity may not gain you anything but a lighter wallet. And maybe more noise. Truly, a YMMV
What appears to be duel exhaust on most modern cars is only duel outlet exhaust. The exhaust from both sides of the engine are collected up front, ran to the rear in one pipe and usually into one muffler that has duel outlets that only give the appearance of dual exhaust.
True duels where the exhaust from each side is ran to the rear through separate pipes and mufflers are not found on stock models.
You say that like its a bad thing. 
this is not the case for a lot of cars with longitudinal vee-type engines. While the pipes are not completely separate (they join somewhere behind the cats with an “X” or “H” pipe) there are two pipes from engine back.
If it’s real it also ***sounds *cooler! And in the old muscle car days you could also use specially designed exhaust manifolds, called headers. They kept the exhaust from each cylinder separate all the way to where the exhaust pipes started (next to the transmission). They allowed for (slightly) better power, but more importantly, by keeping them separate longer they made the detonation sound of each cylinder more noticeable and distinct at the tailpipes. Nowadays you don’t see them, except on classic cars of the era (ending by the mid-seventies), as the sound would just be considered loud and obnoxious today…
Headers are still available for a wide variety of new performance cars. They aren’t popular as factory equipment for a few reasons: Noise regulation, assembly line efficiency (headers typically can’t be installed on the engine before the engine goes into the car, they have to be installed as a separate step) and cost.
On most engines headers do make for a noticeable power boost.
Dual exhaust makes sense on engines that are powerful enough to require large capacity exhaust systems. On a 600 hp engine a single 4" exhaust pipe is hard to route under the car. You’d probably have to make it oval (expensive). Much easier to do a dual 2.5" system instead. Large diameter exhausts are also difficult to muffle and often drone.
I have a completely stock 2005 Cadillac CTS V6 with separate right and left exhausts. So, two catalytic converters, two mufflers, two pipes and two complete O2 sensor systems for each bank.
Not sure if they are there for power, looks, sound, or just to suck more money out of my pocket as part of GM’s designed engineering failures program.
Everything on the CTS is designed with a single goal in mind - to be as ugly as possible.
One thing I leraned when I ‘upgraded’ my ‘71 Charger’s 383-block (punched to around 400ci engine) whas that it was VERY important to maintain the exhaust’s ‘velocity’ to pull in the fresh air/fuel charge from intake (carb’d car). I had planned on going with much larger than OEM pipes (though I did go with dual headers of same diameter (1 and 3/4", IIRC maybe 2" at most), fwiw), but the ‘Mopar expert’ I flew in from out-of-country to hand-port my iron heads (helluva difference!) told me to NOT go larger on exhaust diameter. I put in a custom-made’ ‘exhaust crossover’ as well - kinda a meeting of both pipes just behind engine where exhaust pulses amplify eah other and increase suction to intake manifold, therefore giving a better load of explosive power with each stroke of crankshaft. (On preview, similar to jz78817’s pic-link, but pipes actually touched each other in cutaway fashion, if that makes sense). I replaced the camshaft to accomodate the power-range I wanted - highway mpg was near 40mpg, believe it or not. And it would smoke tires at 30/40-ish+ miles per hour easily, LOL. No one could tell it was a much higher-performance engine since I planned it all to be a ‘sleeper’ drive. Surprised lots of folks, believe me.
The wide-open headers and pipes really sacrifice drivability power-range (and mpg!) and power-range(s) that a street driver is looking for. An engine run at only full-throttle (think drag racers, etc) will not have the problem of sucking in fresh air/fuel -v- lower rpm ranges where the lower exhaust exhaust velocity (at such rpms) pulls in less charges of fuel per cylinder.
There is also the availabilty of having an exhaust ‘dump’ placed just in front of x-over that is either manually or eletrically opened to make it like you have a much larger diameter average exhaust diameter. Kinda like making it an open-exhaust-style like you see on dragsters, etc. Exhaust velocity is critical to performance over powerband desired, IME.
Bigger is not always better for typical driving, IME. Gotta make the efficiency to what the engine is needing at the time. There is lots more I could say, but stock/OEM pipes are often the best way to go unless you have a way to ‘push’ in the fresh air/fuel charge rather than have it sucked in by exhaust manifold ‘negative pressure’, so to speak. If there’s a turbo/super-charger, its all moot, of course 
I hope I explained this somewhat clearly as it is kinda hard to understand at first, or was to me anyways.
The sound was very similar to OEM sound other than a deeper tone due to different mufflers, but performance was GREATLY enhanced by having the highest-possible exhaust speeding out the system rather than it not producing ‘negative back-pressure’ upon the intake/pistons at street-speed power range(s). When I had my exhaust dumps opened (on play-days at car shows, etc), it was terribly responsive at idle/street-driving, but worked great at high-rpms/speed. But going 80-100mph through town was not ideal, ya know?!
Getting the size of exhaust diameter is all a function of where you want your powerband to be at, along with camshaft overlap, etc. A literal science, in a matter of speaking. I a msure Stranger on a Train could use better terminology than I, but I hope it makes sense. Bigger is not always better, not at all. Dependes on purpose of car - used the Charger for daily driver quite often since mpg was so good around town
I’ve noticed that on many “Overhauling” type shows with Chip Foose or others, when they do the dual exhausts, they almost always put in a X cross over pipe, what is the purpose or advantage of doing that?
A disadvantage is the additional cost when it comes time to replace the mufflers and pipes.
Because exhaust gases from one side are now escaping and expanding through both pipes/mufflers (after the crossover). More volume.
most V8s on the road have what is called a “crossplane” crankshaft; the crankpins are arranged so that the crankpins on either end are 180° apart from each other, and the center two are 180° apart from each other and rotated 90° from the end crankpins.
what this means for a 90° V8 is that each cylinder bank has two successive firing events within 90° of crankshaft rotation (if one bank is “Left” and the other “Right,” the firing pattern will be something like L-R-L-L-R-L-R-R.) The “H” or “X” pipe helps equalize the exhaust backpressure from bank to bank when each has its consecutive firing event. you can actually measure performance differences based on where in the exhaust pipes the crossover is placed.
Properly designed headers and exhaust will markedly increase the power in a car.
Headers when tuned to a particular engine can actually increase the torque and move the torque band.
When I was on the race team we were build a new race car. The engine went to the dyno shop. When it came back we were told to have the primaries (the individual tubes) X inches long and the collector (where the four dumped into one) Y inches long
We then installed the engine and brought the header fabricator in.
His job was to build a header in place that had the correct length, would fit in the car, and allowed for the header or engine to be removed.
Obviously store bought headers are for an “average” engine and are not absolutely maximized for a particular engine.
I can guarantee you that in something like NASCAR the headers are tuned for that exact engine setup.
Dallas Jones I can name lots of brand new cars that have full factory dual exhausts from the cylinder head to the exhaust tip.
Caveat to what’s been said: I had a car with dual exhaust, but really it was a pseudo-dual exhaust! It was just for appearances. The dual exhaust only started at the muffler outlet for all to see. Not my idea. It came with the car. (Not a true dual exhaust.)
that’s common for front-wheel drive vehicles. The “dual” exhaust is because the stylists like symmetry.
There is a lot of science involved in how the exhaust affects the power and efficiency of an engine, but the OPs question has a pretty simple answer.
An engine is an air pump. Though most people think in terms of the pistons “pulling” the air and fuel charge into the engine, it is actually atmospheric pressure that is “pushing” the mixture in (on non- force-fed applications). Anything done to make this procedure more efficient and consume less energy is a good thing.
Cars used to have exhaust systems designed for quietness, then later as catalytic emmisions control was incorporated into them, more care was taken to improve flow outside of the restriction of the cats.
Consequently, production exhaust systems have more back pressure than an engine would like to have to fill the cylinders efficiently. Adding “duals” is basically to allow a larger area of exit for the exhaust gases to allow this. While some design considerations do require some amount of back pressure, less is a good thing.
Dual exhaust is one of the few things that can allow an engine to produce more power AND allow better mileage.
A simple example of this would be an installation I did of a dual exhaust “glass-pack” system on my 1966 Mustang back in the mid-70s. The engine was a stock 289 2-barrel carbureted engine, but there was a marked improvement.
True dual-exhaust, and far less restrictive than the stock single muffler system, my average in-town and combined highway gas mileage went from 14 to 17 MPG. There was also a noticable increase in power – maybe only about 10HP – but it could be felt. I could easily run 75 MPH on the highway, with a couple of buddies in the car, with the A/C on, and get about 21MPG.
Dual exhaust reduces back pressure to the engine cylinders, which increases efficiency, so it is a definite mechanical advantage. How well it works has many factors and a lot of science behind it, but the basic premise is sound.