Depends on how the pump is wired.
if direct switched on the panel... on or off. Those you leave off unless you have some water in the bilge. Usually this setup is in a boat where you can see the water at the pump without opening a cover.
It may have a 3 position switch, On, off, Float (or other marking indicating float controlled) Put that in the float position when the boat is in the water unless you know you have a problem and want to force the pump on.
Not common, some may just have the on/off switch but have the float switch. that may be a switch to force the pump on, bypassing the float switch (always able to turn the pump on if water in the bilge) or it may disable the pump completely.
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Main idea is... you don't want to run the pump a long time without it actually pumping water.