Bars have parking lots because very often people use cars to commute to a bar and parking lot is the only place where they are supposed to park their cars. When a group of people walk into a bar it is not necessary that all of them have alcoholic drinks. Some of them might not and they might assume the responsibility of driving everybody safe to their homes. Also, some people have drivers for their cars. Their driver parks the car in the parking lot and later drive them back home.