When this water doesn't drain directly through a scupper or off the deck it accumulates in the bilge. The bilge pump removes this water, either manually or automatically. Without a bilge pump, water within the bilge could eventually cause a boat to sink.
Emergency pumps and installed manual pumps can also work without electricity. These pumps have many uses, from the bilge to wastewater. Emergency pumps set up as a failsafe on boats with automatic pumps installed can be a comfort as an extra safety measure.