The sprinkler system sends a low voltage SIGNAL. It can't send enough voltage to run a pump. The relay just closes on the "signal" voltage and lets line voltage from another source go to run the pump. That is if I understood your question right...

Thanks for the reply.

I guess my question is, if the pump is directly plugged in to an outlet and is able to run on its own, based on cut in pressure and switch off based on cut off pressure..would it not do the same when the pressure varies as the sprinkler draws water from the pump? Just like the pump switches on and off, whenever a faucet connected to the pump runs? Why do we need a pump start relay when it's a sprinkler is drawing water, but not when a faucet is doing the same?

Hope that's a clearer.

Thanks