YOU are in control. No one knows your business better than you do, and no one knows what you want to say, or how you want things to look, more than you. A CMS gives you direct control over the content on your web site.
Security. Many content management systems store your information in a database system where access control mechanisms can more easily restrict access to your content. Done correctly, the information is only accessible via the CMS thereby providing better protection for your site's content from many common and standard web site attacks.
Better customer service. Your CMS might have a public component that allows your customers to contact you directly through your web site as opposed to e-mail. This provides an opportunity to integrate customer requests directly into you customer service systems where they can be handled more quickly and bypass e-mail systems.