The parking brake serves as a mechanical emergency backup brake system. Should your hydraulic brakes fail, the parking brake can be used to stop the vehicle.

Many people who have vehicles with automatic transmissions rarely use their parking brake. They simply put the transmission into park to lock the drive wheels when they park their vehicle. Even so, it's important to use your parking brake periodically.

Using the parking brake regularly helps keep the cables freed up so corrosion can't accumulate and cause binding. Applying the parking brake also works the self-adjusters in the rear brakes, which helps keep the linings in drum brakes properly adjusted for minimum pedal travel. On cars with four wheel disc brakes and locking rear calipers, using the parking brake keeps the threaded self-adjusting mechanisms inside the rear caliper pistons working freely to compensate for pad wear.

The "park" setting on an automatic transmission jams a little rod into the gears. The rod is big enough to hold the car in place even on a steep hill, BUT thin enough so it will break off if the car is pushed or if "park" is engaged while the car is moving.

If you live in like San Francisco, or anyplace hilly, then I'd suggest engaging the parking brake. It works a lot better if you engage the parking brake FIRST, then put it in "P." Less stress on the little Parking pawl, and the shift lever will be easier to get out of "P" the next time.

Hope this explains it.