"The particular wavelength of light that the bulb emits is harmful when it comes in contact with things around it. So when organisms such as bacteria, parasitic protozoa, and algae cells come in contact with the light from the bulb, their DNA is altered, ultimately causing harm and death to the organism. In essence, they are 'nuked.'"
https://www.aquascapeinc.com/water-gardening/maintenance-and-care/uv-clarifiers-vs-uv-sterilizers
After they are nuked they clump together and get filtered out by your mechanical filtration, but if you don't clean your filter pads regularly it will just add more decaying biological material to the system. Mine helped a lot in the beginning but now that my pond is established I could most likely turn it off and I wouldn't notice a difference.