Why not just delete them all? Wouldn’t it save space? Just curious.
I think it has something to do with the Admin’s desire to keep a lot of the message board out in the open and available for viewing by the public, instead of them picking and choosing what we can see.
There are exceptions, of course.
Also, they like to be able to have evidence to point to when someone asks the inevitable “Why was so-and-so banned?” Sometimes, however, a poster does NOTHING but trolling, so deleting his/her posts wouldn’t be missed.
And, on some of the longest threads (five pages and up) where a lot of valuable information, useful discussion, and the like has taken place, the thread becomes too long for the vBB software to effectively manage it, with the potential for corruption. The wise Moderator at that point locks it, opening (or having a member open) a new thread as a continuation with a link to the old one.
Another good reason for locking rather than deleting.