I never used one, until I got a green water outbreak that I COULD NOT clear.
Plants were healthy and algae-free, CO2 was perfect, yet I had green water. 70% WC (100 gal of RO water each time) wouldn't get rid of the green water; it would come back within 1 day.
I installed a 14W UV sterilizer and after 3 days had crystal clear water.
On my experimental tank that has an actual volume of 34 gallons, I have an Oase 7W UV clarifier. It's unlikely to do any sterilizing, but definitely has helped keep the water crystal clear. I mean, CRYSTAL clear. I get GSA on the glass at the end of the week, but the water itself is like acrylic. It's beautiful.
I daily dose micros, and the only chelated thing in the micros is the Fe DTPA. UV Sterilizers WILL break down chelates rapidly, so I run the UV sterilizer only at night, right up until the minutes that the daily doser doses my micros. Haven't seen any issues!
On my bigger tank, I haven't had a UV sterilizer for a while. Water is very clear, especially with a dense-sponge prefilter, but it's definitely not as clear as it was when I had the 14W UV clarifier installed.
In the next version of my big tank, I'm installing a "real" 55W sterilizer, usually used for reefs. Is it required? No. Would I recommend it? No. Does it improve water clarity? Maybe in some situations. Will 55W actually sterilize my water of free-floating bacteria and algae? at the flow rates I'm running, not unless I run it for 8+ hours a day, which I plan on doing. Am I going to write way too many words on scapecrunch about it? Yes!
So I would not consider them even remotely close to a necessity. In fact, I'd recommend finding a removeable in-tank unit that can be installed for cloudy water situations, and removed a few days later. Let me test the $600 UV sterilizer and I'll let everyone know if it makes a difference or not!