There are several types of vehicle garages in GTA games.
The first is a place for storing vehicles, almost exclusively attached to a Safehouse. A variation on this is the large hangar at the Verdant Meadows airstrip, in which larger vehicles including planes can be stored. The hangar at Las Venturas Airport is only used for spawning the AT 400 passenger plane.
The second type is a public vehicle repair business. Pay 'n' Spray garages exist in all GTA III Era cities, and will repair any non-public vehicle, as well as repainting it and replacing the engine to put the Police off its trail, and remove a Wanted Level. Transfender and Wheel Arch Angels are public garages in GTA San Andreas from which Car Modifications can be purchased, such as new wheels or spoilers. 8-Ball also operates a bomb shop in which a remote-detonated bomb can be affixed to any vehicle.
The third type is a dummy garage which is only active for a certain mission, in which the player would deliver a vehicle or some cargo.