A relay only needs a watt or two of power to activate it. 30 is the power circuit input. 87 is the power circuit output. Some relays have dual 87 terminals. In this case, you can use one 87 terminal to power the left headlamp, and the other 87 terminal to power the right headlamp.
Some relays come equipped with provisions for a fuse right on the relay.
Keeping heavy current loads out of this area reduces stress on the entire wiring system, and eliminates much voltage drop on the charging side of the wiring. The alternator need not push the current for the headlamps through the entire wiring harness to the battery, but rather can supply the headlamps directly via the relays. No voltage spikes are present in an electrical system with good voltage regulation, and any spikes that are present in a system with bad voltage regulation are present in equal magnitude across the entire system. The system incorporates fuses in the power supply side of the headlamp power circuit. The alternator can pump out 60 amps or more, and the battery can contribute another 80 to 100 amps before the vehicle main fuse or fusible link blows.