martinibuster - 6:40 am on Sep 18, 2012 (gmt 0)
The idea is to mark threads as solved so other don't need to participate in them anymore.
Good answers so far, great thread. Now a different point that hasn't been mentioned yet. I really don't like the idea of any "feature" that discourages members from posting, particularly new members, no matter how old or "solved" it is. A forum should endeavor to get people posting, not hold them back.
If the new post is off-topic or repetitive spin it off into it's own discussion. If it's on topic then let it continue, that's a bonus for the discussion and your SEO. It seems the folks likely to do this are new members. Getting folks out of lurk mode is a big deal. So why create a feature that might keep someone in lurk mode?
I prefer features that encourage discussion. I understand that the idea behind locking threads is to encourage a new post, but I believe that approach can backfire because it's easier (for numerous reasons) to post to an existing thread than it is to create a new discussion.