Have you ever pressed the gas paddle when the hand brake was all the way up?
Basically compared to the regular foot brake the hand brake is very weak, if it can not stop you own engine who’s says your car will stay in place in case of an accident?
So, let’s say your car is standing in an intersection or a light with as cars rushing in front of you. If someone will collide with your car from behind (and you are not pushing on the leg breaks) you will be hurled in to rushing traffic, which just may kill you!
There you have it, keep on pushing!