Mold can't grow in the sun light. That's the basic idea behind the UV lamp. The dark, damp confines of the furnace duct system, particularly around the dampness of the A coil, provide an ideal breeding ground for molds and other microbes. UV lamps are designed to kill airborne microbial contaminates and break down VOC's and unpleasant odors as they circulate through your home's ventilating system.
A UV light will also help kill and eliminate the growth of organisms on the evaporator coil helping to reduce the risk of illness and allergic reactions caused by these organisms. It's like adding sun light to your duct system. If you or someone in your family has allergies we strongly recommend you consider the UV light.