Tractor Repair: Auxiliary lights, negative terminal, common ground

Question
I am hooking up some lights, and I was wondering why I would need to hook up the ground to a ground on the chassis, instead of hooking it up directly to the (-) side of the battery? It seems that it would be a better circuit, but who knows? I read somewhere a long time ago that you shouldn't do that, but don't recall seeing a reason why. Thanks for any input!

Answer
the negative or ground side of the battery is connected directly to the chassis of the tractor (this is true for cars and trucks as well) this make wiring a lot simpler, by dealing with one common ground, that every circuit on a tractor (or car, or truck) will have a unified ground.
hooking direct to the battery negative terminal will not cause any damage, i have done it many time when it was the simpler option, but going from the chassis ground is simpler and holds a just a good a ground.