You'll get some different opinions about whether UV "helps" to prevent fish diseases. I think it does, and I have used a UV.
However, IMO, they are not really a good choice for a reef tank.. UV kills opportunistic protozoa and bacterium, but they also kill off many micro organisms that are in the water column---something you definitely want to avoid in a reef, as these organism are a primary food source for inverts.
Your best bet for reef, is to QT all fish that go in the tank first. Come to think of it...that’s the best for all aquaria!