Often there may be threads with a certain prefix that are not on the first page, so a list at the top to immediately sort via prefix would be very useful.
Also many users are not eve aware that you can sort by prefix, so if there was a box that not only displays all available prefixes for that forum, but also has the handy instruction: "Filter threads by prefix"
Edit: as Mike pointed out, you can actually do this via the Thread Display Options at the bottom of a forum, but I think that is very well hidden. I didn't even know it was there. I'm looking for something newbie proof and obvious as it's such a useful feature it shouldn't be hidden away like that.