One of the main scopes of a CMS is to manage any kind of content and data structure with as little effort for the users as possible. So it is very important to handle the content with enough flexibility to allow choosing what to show and what not to show in any possible context.
For example, a good CMS should allow us to store a lot of information on an article, useful for internal use, and choose not to show on the site frontend, because it is not necessary for readers. Or, instead, a CMS, gives us the ability to update our articles by adding information and rewriting them easily.
So, we not only need a complete decoupling of the content from the design, but also the ability to manage the content in different possible ways.