Basically, there's two reasons.
The main one, is that when a sale thread has finished, currently people report it and we can go in and delete it. This is all well and good when everything goes well that's great, but it's also a problem should someone get something and feel it's not as it was described. Now, we can move completed sale threads into the graveyard forum so that a) they don't clutter the forum, b) people can't dig up a thread if you've already sold it and c) if there's a problem the thread can be read to see what happened as nobody can edit post or reply in there other than mods.
The second reason, is that by clearing out all threads over a certain time it makes navigating and searching within classifieds a lot quicker and less server intensive. i hope.