All living things need oxygen so it is important to ensure your pond has a good oxygen level. 
What is not so obvious is that the bacteria that form part of the biological filtration process also require a good oxygen level to be able to perform correctly and keep your water to the desired quality.
It is becoming ever more popular for air pumps to be used to raise oxygen levels in filters. This increases the biomass which in turn enhances the overall performance of the filter resulting in better water quality and healthier fish