There are three types of filters that can help keep your backyard pond beautiful and healthy. Biological, mechanical, and chemical filters all have different purposes. Pond skimmers are a type of mechanical filter that grabs leaves and other debris off the surface of your pond. Other types of mechanical filters strain out suspended particles.
Pond skimmers are designed to work with other types of filters, as a sort of pre-filter that removes the larger elements such as floating leaves, which would clog other types of biological or chemical filters. These same floating leaves would also clog the pump. Many pond skimmers are creatively designed to fit with your pond's design.