As far as I know, the only 'safety aspect' of a parking brake is to keep a car from rolling away on a hill. If the hydraulic brakes completely fail, the parking brake may help slow it down over a longer distance though it certainly won't stop the car in a panic situation.
Nor is it meant to- it's purpose is to keep the car stationary. I suppose the term "Emergency brake" is somewhat of a misnomer, I should more properly refer to it as "parking brake".
And yes, the safety aspect is keeping the car where you left it.
It's other purpose is doing "Rockford turns"... :)
