Before answering this question, understand what a parking brake is and how it works. As part of the overall braking system of a car, the parking brake connects to the rear brakes. When engaged, this system applies pressure with less force than the primary system does via the brake pedal.
When first introduced, the parking brake was meant to be a failsafe in case the central braking system failed for some reason. Today, however, this system is used to keep a vehicle stationary when parked, especially on steep slopes.