I'm not sure I understand the question. Drupal is a headless CMS if you want it to be. Do you mean what would be a reason for using Drupal's front-end over developing your own? I came to Drupal from managing community sites with lots of different user roles that each need complex permission sets, and I'd say it really excels over other CMS front-ends for that. Drupal.org is itself an example of this kind of site: thousands of different users (including untrusted users) logging in and creating and editing and moderating content in real-time. Sure, you could rebuild all of this functionality with the exposed APIs, but that'd be a ton of work.

You can easily use Drupal to create the backend for a headless CMS. It's about as simple as turning on the JSON API module that comes out of the box, and then start creating content types.

But then you're having to maintain the CMS, which isn't as appealing as using something like Contentful. It really depends on your use case (like all technologies, of course).

Drupal basically is a headless CMS plus a CMS front end if you expose the web service module.