The idea of a relay is to isolate the switch circuit from the light circuit. It has charged a little now with low amp LEDs. Sometime you can run them without a relay because they draw so few amps.. The relay allows the switch to only have `200ma going through it, and not the multiple amps that lights draw.
In your drawing I would replace your on/off switch with a relay. Connecent the #30 to the fuse block , #87 to the light, and #85 to a ground. Connect a wire from the same +12v to a switch then to #86.