Most of the time, I come to these resolved threads because I see them high up in the thread list, and they don't have a "[RESOLVED]" indicator on them. Without that, I have no clue (I'm not psychic) whether or not the thread is resolved. So I waste time reading through the thread, and I'd like to help others to not make the same mistakes as me. So I tell the original poster to mark it resolved, because, most of the time, he/she will say "thanks", use the fix, and go on their merry way, never thinking about this forum again.
Also, I don't actively look for resolved threads that haven't been "[RESOLVED]". I actively look for threads I can help people in; however, like I said earlier, when I come across one which is resolved, I'll let them know so that A) others don't make my mistake, and skip past "[RESOLVED]" threads, and 😎 the resolver now knows a bit more about the board and how things work.