Josh,
When you use the water for drinking the system needs to be 'cleaned/sanitized' about once a year on average. During that time, bacteria does get more and more plentiful and will sometimes give the water a 'moldy' taste. By running the UV right before you let the water exit for drinking, you kill the excess bacteria that live in the line. It probably still won't stop an off flavor if it gets that bad, but it should be very low bacterial/virus counts.
HTH
Doug
