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Also the purpose of a relay is to separate the high load from the actual control or switch. A starter solenoid is nothing more than a simple relay. Imagine if you had all the starter load being drawn through the ignition switch. Just not practical.

So that is the reason for the relay in a case when you are increasing the load drawn by the headlights, you are increasing the wear and possible arching through the contacts in headlight switch, dimmer switch, etc. This can lead to the contact heating and burning if there are not beefy enough.

The relay reduces the load on the switch to almost nothing, and allows you to run a beefier load (wiring allowing) for the headlights. Also this reduced the contacts/resistance points in the circuit.

Common headlight relay diagram:

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Just be sure the mount the relay in a CONVENIENT FAST ACCESS location! If it fails (like out on the road at midnite, in a rain storm NO moon lite) then it will have to be bypassed with a jumper. Or better yet just ad a toggle sw. near the relay that can be flipped to bypass the relay.

Mite want to add an alarm that goes off when the ignition sw is turned off so that if the relay HANGS CLOSED it will not run the battery down while the car is parked overnite at a remote location.

Carry a jumper wire and a spare relay.

Unwarranted Doom and Gloom aside.....

I've never had a modern relay fail on me, ever. Usually what kills relays is overloading them past there amp rating. This can fry the points inside the relay. But the power feed should be fused, so this should never be an issue.

If you manage to fry a relay, then you would have for sure also toasted the contacts in the headlight switch if not using a relay.

Lets not forget a starter solenoid is nothing more than a relay, and we don't carry jumper wires, bypass switches or setup rube-golderburg style alarms to warn if something goes wrong with it.