I may be wrong but if memory serves me correctly, If an apu is installed on a larger a/c (don't forget i'm used to flying a pa-28), then the apu is used to start all the other systems. This is because the apu can usually be started using a starter motor. The apu uses the a/c's own batteries, whearas the main engines need bleed air to start (from the apu or from a ground source). This is because the apu is much smaller and therefore needs much less torque (hence much less energy) to get going. Once it's going it can get the main engines going as well. Therefore if there is a functioning apu on board the aircraft then much less ground handling is needed to get the a/c going. I can't see when an APU would be used for in flight emergencies. If all the engines fail then i would imagine the batteries and/or the RAT (ram air turbine) would take over the neccesary emergency functions, and not the apu.