The idea of CSS is to abstract design away from markup. One hundred pages on a site may all use the same exact CSS file. This is efficient for many reasons, one of which is that changes to design happen in one place, rather than one hundred. A top-level class can serve to identify which page is currently being viewed and thus apply styling to that a different page may not get.
Different pages, same CSS file, different designs. And because this is happening at the highest level possible element, you can control the entire pages design through a single class hook.