I've only "needed" a parking brake one time, but it was a situation that caused me to include a parking brake in the Bearhawk Patrol I'm building. I was at a fly-in lunch event, and arrived about 10 minutes late – after everyone had walked over the the restaurant next door to the airport. The parking ramp was full, except for one spot that was on what appeared to be a slight incline. I was in a Rockwell Commander 114 (low wing), and after I shut down, I released the brakes (lightly) to see how quickly the airplane would roll downhill. To my surprize, it was rather quick and abrupt. I realized I would have only a couple of seconds to release the brakes, climb out of the airplane (over the wing and behind), then run around to the nose wheel to insert a wheel chock. I rehearsed all the moves several times in my head, then took the plunge. Luckily I did not trip or stumble during the process, and I managed to get the plane stopped a good 3 feet behind the twin parked ahead of me! It took every bit of my strength (and I'm a pretty big guy) to push my plane back up that "slight" incline and into it's parking row. I vowed that when I built my Patrol, I would include a parking brake so I'd never have that anxiety again. (And not the mechanical clunkers, either – a hydraulic valve that holds the brake pressure once they are applied and the parking brake is set.)