CMS (Content Management System) is a system or computer software that is installed into a website builder or a web server to help create, manage, store the content. In other words, a CMS is just an added-in feature for a website where content is one of the key factors in its growing strategy.

That said, while most websites need content to grow, some can absolutely develop well without it such as catalogs websites, ecommerce websites, landing pages, etc. And they definitely don;t need a CMS to build or run a website.