These are clearly aftermarket items. This is not one thing like the antenna, there is a whole row of them. I see them typically on compact imports that have other types of customizations (most of which seem pointless to me).
It looks like bluetooth antenna. I don’t know why they need so many.
I was curious about them myself, only to discover that “shark-fin antenna” is what they’re actually called.
The center fin might be an antenna. The rest are just ornamentation, being about as functional as that wing.
Seriously? People think that actually improves the appearance of their car?
I was thinking it was some aerodynamic shit to improve laminar flow which also would be as functional as the wing, but at least it would have a story.
I believe the center one is the radio antenna.
I don’t know why you would need a Bluetooth antenna on the exterior of a car.
They are another version of “go faster” stripes.
They keep the pigeons from perching and pooping on the rear window.
pidgin?
Thanks! Fixed it.
Agreed, especially because the smaller ones aren’t evenly spaced, either. Aftermarket ornamentation, and poorly installed, at that.
Before I saw the picture (i.e., when I read the thread title but before I opened it) I thought the OP was looking at vortex generators; though how useful they’d be on a car, I don’t know.
After seeing the picture, I think they’re just decorations as Capn_Carl said. (Which reminds me… I need to get a fake car phone antenna for my car. 
)
TL,DR: the claim is that they can delay/reduce boundary layer separation behind your car, similar to what vortex generators do on aircraft wings. This can improve fuel economy, and for high-performance sports cars, it can improve flow over the rear wing/spoiler, resulting in greater aerodynamic downforce - although both of these benefits are likely to be modest at road-legal speeds.
Only $9.37.
Those shark-fin center antennas are for the satellite radio (Sirius, XM, etc.). The standard AM/FM radio antennas have mostly been moved to the rear window nowadays. They look like the strips on the defogger but they’re not actually connected with those.
Or speed holes
I’ve seen similar on semi tractor trucks, dozens of vortex generators up the rear of the cab. They’re apparently effective in that application.
Explaination and photo at the bottom of the article (below the comically boxy truck):
As pointed out in Machine_Elf’s link above, vortex generators as a trend started out with the 2003 Mitsubishi Evo VIII MR. The Evo was a homologation car, meaning Mitsubishi had to build a road car with all of the body work they wanted on their rally car. Furthermore, the rules restrict how high and far back a rear wing can be mounted. You want your wing to be mounted in “clean” air, which means high up away from the turbulence of the body itself, and far back in order to get more mechanical advantage. Both of those are restricted by the rules, so the rear wing had to be mounted too close to the rear window and too low to be as effective as Mitsubishi would have liked.
In this configuration, the vortex generators help to delaminate airflow over the roof and make the air hitting the wing a little cleaner. I have no doubt that this had a measurable effect on that specific car.
There’s no accounting for taste, and stick on vortex generators are both cheap and easy to install. Such is the case with the car pictured in the OP. I will say, while that wing probably isn’t functional, it at least looks sorta like the Evo VIII wing.
This guy probably does…
People will add anything that looks like the features found on actual high-performance cars. Vortex generators, wings, spoilers, low profile tires, loud exhausts, racing seats, the list goes on and on. Of course, it helps if you position them more evenly than the owner of the car in the picture you posted in your OP.