When you install a relay it eliminates having to run a hot wire into the cab to control it. I run the wire for the ground terminal on the relay to the switches in the cab to activate the relays. This way, if there is a short in the wire going through the fire wall, the worst thing that can happen is that the lights come on. Granted, this will happen when a cop is coming the other way, but it "lessens" the chance of blowing fuses or causing fires. Also, using a relay lets you run the wires from the Battery, to the lights over a shorter distance than all the way into the cab and out again resulting in less voltage drop and the need for heavier wire.
Basically a relay is a remote controlled switch. Also, i use the park light circuit for the "hot" side of the switch terminal on the relay. This way if my park lights (dash lights, etc) are off, I KNOW the non highway legal lights are also off. I also tapped into the high bean circuit the same way so my front 100 watt-ers are controlled by the high beam switch. In BOTH cases, the draw on the stock electrical system is insignificant because it takes milli-amps to trigger the relay.
Hope this helps
Fishing the Caribou one lake at a time