INSIDE AN ENGINE;

[AF330]

Hi,
I had a doubt on this, inside an engine, there is a compressor and a com-bustor, but on ground we still give compressed air to the turbine with the APU BLEED, so does the compressor and the com-bustor don't work when the engines are not on? We give compressed air to the turbine but why? During the flight, it takes the air from outside, so why not before start?

Thanks to explain...