The POINT of WHY they use a RELAY is due to COST, pure and simple, a RELAY is CHEAPER than a SWITCH THAT IS HEAVY ENOUGH to carry the same current.
Relays cannot be used in some circuits(not cars) due to NOISE they create electrically.
POWER STATIONS, still use KNIFE SWITCHES, these carry MILLIONS OF AMPS, the sort you see in old black and white FRANKENSTEIN FILMS, they spark like buggery when switching, but on a car it is due to cost nothing esle.
tabetha