Not trying to start a pissing match here but relays are
It's pretty simple really, in the case of a electric fuel pump. A novice might find a keyed hot wire and simply tap into this wire to run the pump. If amp draw is too high and wire not rated for the current draw it could burn the car to the ground. Now if same wire was used to simply trigger a relay it's no problem! That's what relays are for. That's why new vehicles use them for vertually everything. It's also why ma Mopar used a starter relay. When you start adding things to a harness that wasn't designed to have it you need to take that into account