Js/CSS/XML cache prevention. When js file updated, it is not shown for browser in any way. Therefore browser loads an old file from cache. All links to js/css/xml in the site will be realized, for example, as js/<rel>/file.js . They will be handled by mod-rewrite which would remove <rel> from the path. <rel> would be a release variable in database, it would be incremented every time before major update. Therefore for browser it would be as a NEW file, but actually it would be simple old file.js updated.