Originally Posted by tinkicker

it provides ground power and air to cool or heat the cabin before and during boarding. Generally it is much more effective than the ground supplied air used for conditioning.

It also allows the aircraft to be pushed back before starting engines as it can provide its own power and without it additional ground equipment is needed which can delay departure. on arrival it keeps the lights on when the engines are shut down and before ground supplies are attached.

it is basically a small jet engine and most airports limit the use of it to something like 40 mins before departure and 15 mins after arrival.