CityFlyer (or whatever they are called these days) are a UK operator who use the prop brake as standard on their ATRs. This allows them to shut down the left engine completely and stop the prop turning on the right engine while keeping the generator running (as there is no APU) so that the GPU can be plugged in by the ground crew. The engine can then be shut down with a seamless transition of electrical supply from engine generator to GPU without risking chopping off the ground crew's head.
Use of the prop brake on turboprops without APUs is a deeply debateable point due to safety issues and more carriers don't use them than do use them. By the way, on the Saab 340 the prop brake meachanism weighs more tham a male passenger with baggage so when you can only take 34 people anyway you can get a significant percentage improvement in payload capability by ditching it.