[Yetiman]

The usual cause is in the headlight switch. Without relays all the current is drawn through the switch, and there is an arc every time you turn the lights on which causes carbon to build up along with corrosion of the contacts themselves eventually you develop a voltage drop.

By using relays you eliminate the arcing because only a very small amount of current is draw through the switch, and even if there is a voltage drop, it will not effect the operation of the relay.

The contacts in the relay are much heavier duty and better able to handle the current.