This is one of the conventions of UseModWiki and MeatballWiki in particular. See MeatBall:DeletedPage, which we may want to copy/modify here.
One big reason to mark pages for deletion is that the SandBox generates lots of extra pages which will clutter up search results. The moderator can search for and delete pages marked with 'DeletedPage' or use one of the deletion mechanisms used in other wikis to have the users take care of this maintenance.