I've seen the need for Green water algae blooms for new tanks where they add lots of light and do not add enough plants, mulm, no cycling of their tank, or they try fishless NH3 cycling and add high light(very good way to induce a GW bloom).
Generally folks use the UV once, and then it sits after the GW algae is gone.
Some keeping it running, which is a waste and leads to rapid bulb burn out.
Then when you may need it later, it's not as effective or hardly at all.
But it does no harm in running one, but they are not a needed item.
A good idea is to form a local club, then have a group 15 Watt or so UV for the membership to use. Mostly newbies who loused up their tanks with not enough plants, new tanks and way too much light.
I have never needed a UV for that in marine of FW.
The medication or simply placing them in a well run planted or reef tanks took care of that. Lots of water changes also helps a great deal and then you never need UV's for disease management.
I have a nice Current 15 watt sitting in garage. But I have only used it once in sometime.