There basically used for boilers and aircon units to pump away the condensate discharged from them when it’s difficult to fit a gravity pipe system . Say for instance the combi boiler is not near a outside wall adjacent to a waste pipe or grid you would install a condensate pump to pump the condensate a long distance to the source of waste .the pumps usually can pump the condensate upwards day into and over a loft . The pump is a very versatile bit of kit . They come with cut off switches so if the float inside fails they turn off the appliance to avoid overflow of the unit .