I've used WP as an access control middleware on a electronic health records project and I'm currently using it to build a electronic contract SaaS platform. As a concrete example with my e-contracting app, I was able to get account management (changing passwords, deleting accounts, etc.) by using the Ultimate Member plugin. It would've taken me at least a week of coding to build this from scratch.
Thanks for your answer. I would've never thought of using Wordpress for those applications.
Where would you direct someone interested in learning more about how to extend Wordpress with plugins, themes, taxonomies, custom types, etc.? I'd mostly like to use it in CMS-like roles for personal projects.
I've learned how to build and extend plugins via WordPress.org. I've also spent a lot of time just digging through the source. Combine that with SO and that's been how I've come up to speed. I do remember a good book from back in the day called "digging into WordPress"