What CMS allows you to do is essentially separate all content from design. What that means is that they don't depend on each other. You can change design with out influencing content and vice versa. It also provides a structured form of administration to the site allowing more flexibility and sophistication per say. For example, one of the most popular CMSes out there is Joomla. Joomla comes with an excellent back-end administration that allows for the quick and user friendly administration of many tasks.