I use the parking brake:
- when parked, always, no matter whether I'm driving a manual or auto
- to keep from rolling backwards into the jackass who has crept up to my rear bumper on a steep hill, when launching a manual
- in conjunction with engine braking if I want to slow down without lighting the brake lights

Related question: if you have a manual, assuming you always use the parking brake, how do you leave the transmission when you park? I leave it in 1st; I've met others who habitually leave it in neutral or reverse.