The Nautilus emergency float valve shuts off your pump automatically when it detects high or low water levels. An audible alarm sounds when the float is tripped.
This is useful for protein skimmers- when the collection cup fills it will shut the pump off and prevent overflows.
In sump it is useful for protection from low water levels- the pump will fully shut off before it begins drawing in air. (Nautilus pumps have additional dry run cutoffs built in- and will not run dry with or without the float switch).