The sump becomes a bio-filter with a bit of time. Gunk and critters from the main tank get into the sump and settle out of the water and onto whatever subsrate you have there, ls, lr, both, macro-algae, whatever, and it biulds up the bio-filter. Then there is another compartment, depending on your sump configuration there could be several, where the return pump, skimmer,heaters and whatever other equipment you don't want in the main tank go. Supms are an easy place to add top off h2o, test, add suppliments and even water changes. I'll never have a tank without one.