Electricity loses voltage as it travels through wire. The larger gauge and shorter the wires, the more voltage at termination point. Relays allow for a shorter run in some cases. The biggest benefit is replacing the stock 16-18ga wire with 12ga wire. This provides more voltage at the headlights. Headlight performance is not linear with voltage drops, it's exponential. A 10% drop in voltage can result in a 17-20% decrease in light output. Hook a 12 vlot light up to a 6 volt source, it won't likely be putting out 50% of it's output. Probably barely warm the filament.