The parking brake is an independent / redundant system. After the hydraulics have fully failed (which, no matter how well designed and built you think the system is, can still happen: https://en.wikipedia.org/wiki/United_Airlines_Flight_232 ), the cable actuated brakes can still serve to get the vehicle stopped more quickly and safely than opening the door and dragging your feet on the ground.
The parking brake is an independent / redundant system. After the hydraulics have fully failed (which, no matter how well designed and built you think the system is, can still happen: https://en.wikipedia.org/wiki/United_Airlines_Flight_232 ), the cable actuated brakes can still serve to get the vehicle stopped more quickly and safely than opening the door and dragging your feet on the ground.