trying to understand when to put relays in

[71dustar]

Do I need a relay for every extra switch I add to an electrical system, (electric fans, electric fuel pump, ignition (keyless start), starter button...)? I know I should put a relay for headlights and there's already one for horn.

 

[74fldart]

if the part is going to draw hi amps you need to , otherwise you will need to run a large wire to each accesory. the relay allows you to run a small wire back to the switch and run the large wire only to the relay and the pump, fans , etc.l

 

[RedFish]

Most manually operated switches aren't designed to cunduct high current.
The can signal a relay with very little current. The contacts in the relays can carry high current. You save switches, length/gauge of conducters, and gain reliability.