Sponge filters provide both mechanical and biological filtration. Simply install an airline into the unit and provide both aeration and filtration for your fish. Aerobic bacteria colonize the sponge providing a bacterial substance to facilitate the nitrogen cycle. In addition, the sponge traps larger waste particles mechanically cleaning the water without trapping baby fish. 

They are useful for as breeding filter or as secondary filter.  Or they can be used as pre-filtration unit by fitting to an external or a top filter. These units will last you for years and will not need to be replaced for a very long time.