Basically, it is a float switch which triggers a light, and/or an alarm, instead of a pump. If you don’t already have a secondary bilge pump, it would be a good idea to install one, and include the alarm in that circuitry. If the secondary pump has a built-in float switch, you might want to install it a couple of inches higher than the primary, if the float switch is separate, install the float switch a couple of inches higher.
It is a good idea to have a light, or an alarm, on all of your pumps, in case your primary is running constantly, or frequently, due to a small leak. If you are at the helm for an extended period, like running to an offshore spot, you might not notice the bilge pump discharging aft, and that small leak could turn into a flood.