They mostly delete them because the situation that was on the forum post was solved and since it was settled there is no reason to have it there so they trash it
Lol... I never really got that xD Why do they keep clearing everything and keep the forums so small with stuff? Mhm...
A lot of threads will be moved to the Archive, this is to keep it neat and to know what we have done and what's left to do. Topics and/or posts will be deleted for a lot of different reasons. Most if not all forums have an archive, posts are only temp hidden, so moderators can still see them if they ever need to be restored they can.