lol, I actually don't have a great reason for this, but (thinking about it now) it is something I do all the time.

Usually headless in this context refers to content being exposed through APIs instead of a generated user-facing frontend. This way, you can develop the user-facing frontend in whatever frontend libraries and frameworks you want and just consume and expose the CMS content through APIs, and you can use the same APIs to expose content in mobile apps etc... A typical example of a non-headless CMS is Wordpress where you get the admin panel for content management, _and_ a user-facing frontend for end-users to consume the content.