Plumbing and Heating

Why does it seem that all plumbers are also heating people as well? Am I missing something? Are my plumbing and heating systems somehow one and the same? What part of plumbing is also heating? In fact most of the time they are Plumbing, Heating and Air Conditioning. Does my plumbing run through my heating and A/C lines or vice versa? When I flush is there a way to turn on the A/C. I don't get it, please explain!