It slices it dices, cleans water, maintains Ph, removes nitrates, makes fish happy, exports DOCs, makes teeth whiter and brighter and gives fish a general feeling of happiness....
ok, doesn't do all of that... but it does remove DOCs (Dissolved organic compounds) its main reason for being. Remember everything you add to the tank stays in the tank (forever) unless you removed it (ok water does evap). All tanks need a excess nutrient export. Water changes, algae harvesting, Skimming, denitrification via DSBs and LR are all methods of export that work. If you don't have a skimmer and have acceptable readings then you don't need one. If you are battling nitrates.... might be worth considering...