As a complete newbie I find it useful to browse solved threads and find solutions to problems, the best thing is that often there are multiple solutions provided and you really get an "on the field" view of how to do things, rather than a technical knowledge that may not be most suitable given the situation...

Some members, experts, seem to like solving threads and may not necessarily want to look at the bulk of solved threads already.

So it would seem being able to sort by solved threads, or unsolved threads, could be a useful feature for large groups of people.