[jehb]

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.