Protein skimmers remove basicly any organic waste products from the tank, protein, amino acids, fats and oils, bacteria, free floating algea and some parasites will all be removed by a skimmer, they will also remove any medication or additive used in the tank. The benefits are that longer periods of time can be left betweem water changes and nitrates will be lower as the proteins will have been manually removed rather than converted to nitrate by the biological filter.