Some people do it to reduce (or eliminate) the need to clean the grill as often as they would have to if the grill wasn't covered. Covering the grill with a light weight aluminum foil will, in many cases, allow the grill marks to develop so it's not entirely a loss. Aluminum foil also collects many of the fluids from the food, preventing or at least reducing flare up.
I use aluminum foil alot while using the bbq- even on my propane bbq- for clean up reasons, for flare up reasons, for drinking too much beer and not wanting to burn food reasons mostly!!!!
I make alot of hobo packs too- so sue me!
If you have really juicy or marinaded foods- it really helps with the flair ups.