Sumps and refugiums can be used in order to increase your total water volume which also helps in diluting your water from accumulating harmful pollutants. Sumps also provide a handy "hiding place" for other necessary equipment you don't want hanging around your "display tank" such as heaters, protein skimmers, grounding probes, and such. A sump also keeps the water level in your aquarium the same at all times, the evaporation still occurs, but it is generally out of the sump instead of your tank.