Garages are infrastructure buildings in Tropico 3 and Tropico 4. When connected to a road system, garages allow citizens and tourists to get into a car and move around the island at a much faster rate. This makes it a good idea to put garages near housing and other such buildings that don't have built-in spaces for cars.
If too many people use a single garage at once, the garage will become too full of parked cars. In very busy areas, it might be a good idea to build multiple garages.