Many organizations are wise enough to know that they need to keep their site current and fresh. So, they collect changes they need to the site and they periodically call up their web developer, who, in turn, makes the requested changes to the site. This process is slow and expensive. There's a lead time between when the new content becomes available and the web developer is called, and between when the developer is called and the changes are made. The developer typically charges by the hour for the modifications, which adds up quickly.
Some organizations make the changes to their web site themselves.
This process can work when somebody has been properly trained and is careful about the changes they make. However, all too often, the staff person maintaining the site unintentionally makes adjustments to the site's overall look and feel. As time elapses, the site gets uglier and uglier. Or at least not nearly as often as they should. A Content Management System is software which allows staff people to add, delete, and modify their web site's content easily and directly.