It all depends on what you want to present online. Apart from the form and design, the most important thing of a website is its content, naturally. This means that on most of the sites the content shall be changed and upgraded, which is the only way to prevent the site from getting obsolete and uninteresting for visitors. However, changing the entire website for each particular modification is rather impractical; therefore only the content and media should be changed. With CMS websites you can do it yourself, quickly and efficiently.