[newnewpdro]

The real problem is you're not loading (and testing) the brake before putting the engine in gear.

An off engine is not unable to turn. Its resistance to turning is not constant either. All piston rings leak, even when new. So while you may have observed the engine in first gear holding the vehicle, minutes later the leaking piston rings may have allowed the crank to turn, and vehicle to roll.

The primary holding mechanism should always be the brake, and you need to load it to verify it's functioning and that you've pulled hard enough on the parking brake.

Putting it in gear is just a weak and largely ineffective form of redundancy.

Chocking turned wheels against the curb is much better redundancy (and legally required when parking on hills around here).

[lopmotr]

Though the engine won't stop it rolling away on a steep hill, it will keep the speed down instead of wildly accelerating. So that can reduce the danger and destruction it causes.

[asdff]

Once you are off the slope it will stop the car as well

[eecc]

Very well summarized