UV sterilizers are optional. Lots of people love using them. They do help cut back on free-floating stages of disease and algae spores, but they will do nothing for any algae or disease that is not floating in your water (i.e. on the fish, on the rocks, on the sand, on the glass). They also will kill any good things that go through them, so you'd have to weigh which is more important to you.
I think they are a great idea for tanks that have algae or disease problems, or for people that keep disease prone fish (like a lot of tangs), but they just aren't necessary for most reefers.
Also, using one for algae or disease doesn't get to the root cause of those problems, it only treats the symptoms.