the skimmer uses small bubbles (the more, the better) to remove wastes from the water. there are several different skimmer types (venturi, turbo injected, needle wheel, etc) that put bubbles into the reaction chamber in different ways. the longer the bubbles are in the water, the longer they have to contact waste, and therefore the more waste they remove. very (VERY) simply, the bubbles are created, go through the reaction chamber where the waste comes into contact with them and sticks to the bubbles, and then the bubbles form a foam that rises up the neck and spills into the collection cup and turns into a nasty liquid (skimmate) to be discarded (or recycled into a refreshment for a friend that owes you money). i believe that needle wheel skimmers work the best. as far as a need for one, i'd say you should get one. with enough lr and other forms of filtration you could get away without one, but it's better to have one and not need it, than need it and not have it. flitration is the most important part of a tank. without proper filtration its tough to keep anything.