Where the content lives
Where your content lives is a separate decision from where the app runs. For a site that is mostly writing, membership and email, a content platform earns its place rather than building all of that yourself: content sits apart from code, so publishing is not a deploy and an Admin API still lets you automate from scripts when you want to. Ghost is a strong fit here because posts, member accounts and newsletters come built in; WordPress or a headless CMS such as Sanity are other common choices, with different strengths. Where content is tightly bound to bespoke app logic, or needs a very custom data model, a CMS can get in the way instead, because you are taking on its model and its hosting.
Say it another way
A CMS, a content management system, is a ready-made place to write, store and publish content, with the editing tools and the reading experience already built. Ghost is one aimed at writing, membership and newsletters.
Part of Designing the system.