Riddle Me This

So the parking brake is supposed to be the super-strong brake, right? As in, if you're on a hill and you need extra brakes, you pull out the big guns, the parking brake.

So, if the parking brake is so strong, explain to me why I can drive all over town with it on, without even noticing. I don't think the parking brake does anything. I think it's a sham. We're all being had, people.