1. So you can reach in and turn off the master switch after checking the lights, stall warner and pitot heat and before the battery is too flat to start it on a cold day.
2. So the dwarf sitting in front of the nose wheel not wearing a high vis jacket can hear your 'clear prop' call.
3. To clear the cabin air quickly if you frighten yourself, your instructor or your passenger(s) during a critical phase of the flight.