It depends on what type of forum you own really as for most forums prefixes aren't really needed. Forums like FP that are promotion based are fine to have prefixes as it can help with organisation and help people find the topic that their looking for. I'm thinking of actually creating prefixes for my upcoming forum as well that are exclusive just for upgraded members
It depends on the forum really if it's a general discussion forum no because there is loads of different forums for things like sport and music. But it could be on a gaming forum, say it was a GTA section you could have prefixes for each GTA game or which console it's on. A promotion forum definitely because it would be easier to separate the forum software or looking to see if a thread is open or closed.
Some are used to help find threads on a particular game series. They are very useful and prevent the need of making a load of dedicated sub-sections/forums. 😛
Prefixes are everywhere on my forum depending on the board. 😛 From bugs to feedback to even the announcement board, I find prefixes to be an organizational tool to the forum (especially when you can click on them and instantly be redirected to a search page) and it adds more color to the thread title.
For some reason I just like having prefixes, it's one of those strange little things that gives me that "official" feeling when stamping a prefix onto a thread.
No, they don't really fit the niche that well, so I haven't seen any reason to use them. But I do understand why some might, and should, use them on their websites.
Yes on most of the forums I have owned I have implemented them. They help separate content a lot and stop the need for various sub-forums which I don't really like.