Skimmers can help remove debris from the surface of your pond. Typically the pump is in the skimmer box so the suction from the pump creates a circulating current that brings debris on the piond surface into the skimmer where a net catches it for easy removal. Skimmers are not required in every pond absolutely but they are an easy way to keep dirt, pollen, leaves, needles and twigs and flower petals from getting in to your pond and sinking where they decompose. Smaller water gardens can often be skimmed with a hand-held net or pool skimmer but using a specialized skimmer with brushes to catch debris is an easy way to maintain a clean pond surface. Make sure to follow directions and install the skimmer correctly according to the instructions provided to avoid danger to your fish.