Aftermarket headlight relays were a VERY popular addition to cars in the 1940's and 50's, particularly on 6 volt systems with slightly corroded connectors and rusty ground circuits. Dad's mechanics installed dozens of them durng that time. People were amazed at how much brighter their headlights got.
Jerry
It's still popular, because the manufacturers (Ford in particular) use overly small wires from the fuse box, to the headlight switch and back up to the headlights. That causes a significant voltage drop and the headlights see lower voltage than they would if sufficiently large wires were used. Also, the switches are less likely to burn the contacts if all they're switching is a relay coil. I can tell you that headlight relays work.
