Sometimes in an organisation you have an app or webapp where the same staff members who write content and marketing posts want to be able to write content for places in said app/webapp.
Instead of having those content changes be part of the change request cycle between Engineering and other parts of the organisations, they can write an CMS integration into their app/webapp and allow that department to make changes fast without involving engineering and utilising their existing skillsets.
It also allows said external departments to use the tooling they use for blog posts to enforce approval workflows and content policies.
Like anything, there are tradeoffs.