The easy answer is simplicity!
Content Management Systems (CMS) make it easy for you to have total control over what, where and when things happen on your website. It puts you in the driving seat of your website and allows you to perform updates and changes on your site and often with no more complexity than using a word processor. It does of course offer a whole lot more besides but simpliciy of making changes is a key feature.
What else can it offer?
The list of easliy incorporated features is too long to detail but some of the standard features that are often required include:
Secure client logins to restricted and private members only areas
Targetted content - feed relevant content to particular users
Self or restricted registration capabilities
Web forums
Versioning for content and documents
Email newsletter management features
Search facility
Easily created dynamic content like: RSS Newsfeeds, web polls and surveys
And of course the on-going cost savings are significant. No more paying for simple website changes or updates; no more excuses for out of date content!