My collaboration with DentaQuest, the largest dental benefits solutions provider in the USA, started with a vision (theirs): helping people to monitor and to improve their dental health not just anyhow, but as threy efficiently managing all the costs involved, as well.

And it continued with a set of challenges (but I'm drawn to challenges!) that their leveraging Drupal on their portal involved due to all the functionalities and features required.

My Part in this Project:
… consisted of tasks involving Drupal site building, new Drupal theme implementation and, of course, custom Drupal module development (since the portal's an entire "blending" of functionalities and complex features).

Why Drupal?
The portal, due to its heavy infrastructure of patients/visitors and dentists promoting their services, and to its content (oral health-related information), too, depended on a powerful and highly flexible content management technology.

Patients had to be able to easily get in touch with the DentaQuest team, so a highly secured messaging system was one of the top priorities. Moreover, website visitors' quick access to the informative content and to all the dentists' reviews and credentials was crucial.

Also, content on the topic of oral health needed to be uploaded and edited with great ease. So an improved editorial experience was added to the "equation", as well!

Dentists had to be enabled to easily promote their services on the portal, therefore to easily manage their own content, as well.

Not to mention the need for a streamlined process of entry of authorization and  member eligibility check-up, too.

And here's the answer to your question ("Why Drupal?")! In the context of a complex hierarchy of users, multiple permissions and user roles (especially "roles") Drupal  makes the most suitable CMS solution!