I have been reading about the Normandy invasion.
Why deliver troops by glider instead of parachute?
Can a powered aircraft haul more men in gliders than it can carry paratroops?
Why not carry paratroops in the gliders?
They were silent. Plus other reasons I’m sure I’m not qualified to explain.
Bringing troops in by glider saves you the time and trouble of having to train them to use parachutes. Plus a glider is more controllable than a parachute (especially those in use at the time) and so you can better pick a landing spot rather than the paratroopers being scattered hither and yon. The glider troops also get to land with heavier equipment rather than being lightly armed like the paratroops were.
One large issue with paradropping is that you’re going to be dispersed over a wide area, and have little control over where you land. With gliders you have the ability to have a greater concentration of force available immediately after landing. They were replaced fairly soon after WW2 with helicopters, which give you even greater control over where you’re landing things.
And the pilots were not very well trained, they lost a lot of people in crashes IIRC.
They could not be towed real fast so they were very vulnerable for deep penetration. They kept trying to do that and overall were not as effective as hoped.
This is probably the biggest reason; paratroopers are some of the most highly trained soldiers in any army, since they’re trained to safely jump out of planes into enemy territory with somewhat less equipment than your average infantry unit, link up with other paratroopers and fight unsupported for some short, but indefinite period. This means that paratroopers and their officers have to have a higher than average level of self-motivation and soldiering skills, as well as the specific parachute training.
Glider troops, by comparison aren’t particularly different than regular infantry- in the British Army, they were regular line infantry units merely delivered by glider, and in the US Army, they were sort of like the red-headed stepchildren of the Airborne units- not allowed to wear the berets or jump boots at first, even though they were in the same divisions.
Plus, with a dozen or two (depending on the glider) soldiers in one craft meant that they were landed as units, not as individuals who had to regroup and reorganize. So you’d have larger glider units fighting initially vs. paratroopers who were often meeting up with other singletons until they coalesced into larger units.
Helicopters were the sort of spiritual successor to the gliders, even though there was a stretch in the late 1940s-mid-1950s when there were neither gliders nor helicopters in wide use for inserting troops into enemy held territory by air.
They were drawn from regular infantry regiments, but formed up as the Airlanding Brigade of 6th Airborne Division, and trained for glider assault. It’s not like they were just loaded into gliders and sent off with nothing but a cheery “Good luck, lads!” 
Gliders also delivered a lot of payload. Jeeps, cannons, heavy arms. My wife’s father was in the glider infantry, and was a bazooka man. They carried a trained and designated pilot, as well as a regular soldier that had enough training to be an emergency “co-pilot”.
This, I think, was the main driver of glider troops. You could land a concentrated group of soldiers (with heavy equipment!) in one spot for an attack. Paratroops can’t do that. They are great for mass attacks, but if you need that bridge captured immediately, gliders were the way to go.
Along with troops, gliders carried equipment that was too big to drop by parachutes. Jeeps, for instance.
Thanks, all.
I can’t find a picture on line, but I recently saw in a military museum a diorama of a small bulldozer specifically designed to be delivered by glider during WW II. It was cute. The operator didn’t so much sit on it as straddle it. 
The card talked about engineers being sent in by gliders to help with building the initial fortifications for paratroops after a landing.
There are a couple of other factors: gliders are cheaper than planes, and a lot of the drops on D Day consisted of a plane with its own paratroops on board towing a glider with troops on board, effectively doubling the capacity and utility of the plane.
There were engineers delivered by glider in operation Market Garden, to build river crossings.
Things aren’t going well for them in the book Crossing the Rhine.
Here’s one example.
That’s the one I was trying to find a picture of! They had some shots of one in a copy of Military Vehicles magazine a few months ago.