SOME PEOPLE THINK SELECTING “Park” (in an automatic transmission car) means they don’t need to use the parking brake. It may be partly due to the current terminology that calls the parking brake an “emergency brake”.
Once you know how Park works in an auto gearbox, you’ll understand why engaging the handbrake, parking brake or emergency brake makes good sense.
While “Park” does disengage the engine completely from the drive wheels (just like selecting neutral in a manual gearbox car), it doesn’t apply any brakes. Instead, it locks the drivetrain using the small pin (called a “parking pawl”).
Basically, you’re relying on a piece of metal about as long as your finger, as thick as a $2 coin and about as wide as your fingernail to hold your vehicle. And since modern vehicles can weigh from one tonne to many, the stress on that small piece of metal is considerable, especially if you are parked on a slope.
If it breaks, you could be up for considerable (unnecessary) expense: damage to your vehicle if it rolls into something damage to whatever it rolls into; and removal and dismantling of your gearbox and replacement of the parking pawl, or possibly replacement of the entire gearbox.
How can I avoid all of that?
All it takes to avoid the possibility of all that damage and expense is to use your handbrake. It only takes a second.