Every time I edit a post or page, WP keeps a copy of the old version in my database. It is a great feature, handy when I mess up and need to revert to the previous version. But once my post or page is final, I have no use for the prior revisions. By default, WP keeps all the old versions, forever, and they can add up over time. I recently checked one of my sites and was surprised to find 3,566 useless old pages cluttering up my database. A large, cluttered database slows my site, as the server takes longer to retrieve information.