I am new also, and without going into any specifics, just because I don't know them.....I believe it is the heat. MH lights produce a lot of heat, and I think chillers prevent you from having a fish fry in your living room, or where ever you have your tank.
But I have also heard/read that it depends on the location of your tank....like I have mine in a room that does not get overy hot...so I won't have a chiller, but will have fans running to ensure I am not cooking everything inside of it.
With that being said...what is a safe distance you should have/hang your lights from the top of the aquarium? (more specifically hanging, as that is what i'll be doing with my MH)