Tractor Repair: Auxiliary lights, amperage, chassis

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
Always put the ground as close to the light as possible. You do not have to run it all the way back to the ground. i would advise to run the circuit through a relay as to keep the amperage down on the circuit. If the amperage is high the creates heats.