Relays vs. none

What's the advantage of using a relay for aux. lights vs. wiring straight to battery w/ an inline fuse? I know this is probably a dumb question but I was just wonderin.

 

Ok, if you wire direct, you need to pass the entire current through a switch. Such high-current switches are available, but not common.

With a relay, you can control the relay with a light-duty (ie small) switch and let the relay handle the high-current switching. High capacity relays are fairly easy to source, and this keeps the high-current wiring to a short, straight, simple path and out from under your dash. Short, fat wires mean less resistance and brighter lights.

For headlights, a relay allows you to use the original light switch above its rated capacity for big-bulb halogen headlights. A shorter, fatter wire to the lights (bypassing all the wire from the dash to the switch to the lights) also makes your lights brighter.

 

In addition to just straight current path, my light duty switch that I inherited with the JEEP (20A advertised, but I doubt the claim) would arc everytime I turned the halogens off and blow a fuse. When I'd turn the switch back on, no headlights.