Flow controller is completely unnecessary. Haven't used once since the old dial days. Present pump is over 5 years old.
I think the primary purpose is to save water, rather than reduce wear on the pump.
Generally speaking, increasing the flow will speed up the cleaning, but beyond a certain point, increasing the flow will not noticeably increase the speed (so you are just wasting water)
Plus, a flow controller will automatically adjust the power to the pump to compensate for a higher level to pump to, longer hose etc. without a flow controller, the flow would decrease at height and hose length.