It saves installing a seperate switch for every component, not all relays are the same a simple one can be n/o or n/c depends on what you want to control some have times in them or current or voltage sensing as well. as a guide terminal 85 is earth 86 is the signal feed used to activate the coil, 30 is perm feed, 87 norm open 87a is norm closed. Hope that helps a bit. Also using a diode relay helps when back feeds become an issue.