Having the switch to kill the pump isn't a bad idea as YR mentioned. Engine dies and the pump shuts off in an accident, etc. You could use the same switch to kill both ignition and pump.
My guess they matched the breaker to the rated max current of the relay. An FYI, always match the circuit protect to the expected max load plus some overhead. That way you aren't melting things that only need 5 amps and have a feed capable of 30 amps. That's bad engineering. :)
