I've used them when I park too. I haven't used them for starting up hills in conjunction with the clutch. I release the regular brake and release the clutch as I increase the accelerator pedal to start with no roll back.
This works on a slight incline but not on a (very) steep one. By the time the foot that left the brake reaches the gas pedal you already started rolling backwards. Good luck in narrow streets of south-european cities built against hills.
Of course most modern cars have hill assist doing a ~hand-brake start for you by keeping the foot-brake down for you.
Many cars don't even have separate parking brakes any more. The electronic parking brake mechanism is simply integrated in the rear disc brake callipers. The same mechanism is then used to implement the hill-assist.
The advantage: lower BOM and less space required; no separate brake, no steel cable. Also more fool-proof because it automatically disengages.
I don't really like them. The auto (hill) release waits a fair bit until you build 'pressure' and then lets you go with a jolt. (at least in the Volkswagen group rentals I encountered)
The manual handbrake allows for a more subtle hill start release.