In games, parking lots usually serve two purposes: as a place where you can get and store cars, and a battleground. In real life, parking lots are places where you park your car. A parking lot is usually found underground or in a parking lot building. When you enter the parking lot, you must get a ticket so you can pay when you leave. Parking lots usually have different fares and some (especially the ones at the airport) let you park 30 minutes for free. Parking lots have different floors and often have background music playing in them.
In open world or racing games, parking lots let you choose which car you want to drive. Usually the game does'nt make you walk through the lot, trying to find the car you want (only Heavy Rain does this). You usually walk up to the parking lot (or select the parking lot option from the menu) and choose your car.
In some games, the parking lot is used as a place to store cars (especially in open world games). You can drive a car that you like back to the lot and it will always be there for future use.
A parking lot can make for an epic battleground. It's a tight, closed up space that has plenty of cover. The cars in the lot can usually blow up, causing massive damage. Some games like Infamous
have you climbing around a destroyed car park, avoiding falling cars in the process. Other games can have you driving around the car lot and usually once you get to the top of the parking building there's usually a big ramp that you can drive your car off.