A good CMS system will have the navigation structure built into it. This will ensure that as pages and elements are added to the website, the navigation portion will automatically be updated by the CMS. This saves the developer a lot of time creating special graphics for the buttons since it's all done automatically.
Because the CMS is tied to a database it opens up a new world of opportunity. Things like user names and passwords can be stored, different access privileges can be assigned for private "members only" areas, e-commerce systems can be built on top of CMS's, and blogs, event calendars, forums, and photo galleries can be easily created.