well, they have an on off switch but soon as you take away power you have to turn them on again. not so, for example for your aerator for your live wells or bilge. they just come on automatically if the switch is set to on.

and if they're on the same power circuit as the engine ignition you want to turn them off when you crank the motor because of the reasons listed by Thumper and others above. you run the risk of burning out the little naughty bits in them if you do.

if yours doesn't do this , fine.
you're way ahead of the game.

bKB