Relays do one of two things. Sometimes a relay uses the signal coming in on what would potentially be an overloaded circuit to simply trigger a secondary circuit more capable of carrying the load. Your engine's starter solenoid is an example. And sometimes a relay is used to cause a second event to happen at the same time as a first event, with no high current handoff being involved. This is what happens on many Kawasaki motorcycles whose headlights turn on when receiving a signal from the alternator. The headlight stays off until the engine is running, a way of maximizing power to the electric starter. Hondas use a dual-circuit start button to do the same thing without a relay.

Due to its huge requirement for electrical power (a vintage Honda's electric starter draws from its battery more than 70 amps of electric current. That is so much electrical rush that the poor little handlebar's starter button would be instantly fried if it had to carry it; the size of its contacts limits its electrical ability to below 10 amps. Pretty big difference. Thus we have "starter solenoids", actually just special-purpose heavy duty relays.

Powersports manufacturers have used relays for a long time. Yamaha during the 1970s was famous for having a lot of relays on its bikes, but today all the Big Five brands have many relays on their products, for everything from headlight control to fuel injection power, and of course as the basis of all the safety circuits found on motorcycles today.

Forums are advocating the use of a relay for a special reason. Vintage motorcycle electrical connectors have no seals to ward off dirt and moisture, thus are prone to building up dirt and corrosion that makes them build electrical resistance; their ability to conduct voltage decreases with age and exposure. But even connectors in perfect condition exhibit the characteristic of using some energy. And few circuits on your bike have only one connector. Even the horn, the simplest electrical circuit on the bike, has at least four to five connectors, and many circuits have twice that. Even a fairly simple headlight circuit can drop 2-3 volts between the battery and the headlight, resulting in some loss of lighting efficiency. User forums all over are buzzing about the fitting of relays in efforts to recover this lost voltage and thus maximize the designed-in lighting potential. It's not a bad idea either. But at the same time, it bears thinking about. Maybe the actual problem should be addressed instead?