A CMS makes it easier for anyone to create, edit and publish content on a website. Historically, website publishing has required significant technical skills (HTML, programming). A good CMS allows non-technical authors and editors to easily and quickly publish their content.
A CMS reduces time-to-publish, allowing you to get content published faster. This is an important issue for the modern organization. The quicker you get key content published, the more value it creates.
A CMS allows for the design of a common and consistent information architecture (metadata, classification, navigation, search, layout and design). Inconsistent and poorly designed information architectures plague many websites.
Security is an important issue, particularly for intranets. A CMS can facilitate better content security. It can control who is allowed to publish to the website, and who is allowed to see what content.