Originally Posted by
CatchThePigeon
And what are the effects if it isn't working?
Edit - I see that
tinkicker has preceded me.
The APU provides power to a heap of systems on the ground when the engines aren't on, including the air conditioning packs. Without the APU, and in the absence of a PCA unit (basically a big aircon unit with large pipes) the cabin atmosphere can get positively roasty on board, especially in places like MIA.