I think Ryan's answer hits on a lot of important benefits of Craft as a flexible platform, so I'll try and focus on why it's a good choice due to its architectural decisions.
Craft is based on Yii 2, which is an application framework not unlike Rails or Django—although they naturally differ in their handling of MVC principles.
I'd recommend at least doing some of your application design in a platform-agnostic way, and then evaluating frameworks based on those needs. Some may be fulfilled by Craft, some by Commerce and other first-party extensions, and some by third-party plugins. The final piece is figuring out how those interact (if they expose Services you can interact with, if they have a robust Events system, etc), and mapping out what you expect to build custom.
In my experience, Craft has absolutely everything you need to develop complex applications, simply by virtue of being able to drop directly into Yii's capabilities. When you're using it in this way, you'll discover that Craft becomes increasingly helpful when implementing trivial things like Authentication/Permissions, Email, Tasks, or Asset management.
Perhaps the greatest argument for using Craft in light of other application frameworks (granted many solve the same sort of CRUD problems) is that you get those content tools "for free." In other words, once you've got your application built out, you can really easily just stub out a marketing site, and have your client manage their entire web platform in one place.
I can't speak highly enough about Craft being used in these situations. As you're auditing Craft, I'd recommend looking at a couple of first-party plugins to see how they've structured them—there's a bunch of useful information buried in there that hasn't really be synthesized into the documentation, but may help map out some paths through the Model, Record, Element, Service, and Controller systems.
Overall, when I look at what we've been able to do with Craft and compare it to the custom applications I've dealt with in the past, I'm relieved at how little technical debt we've accrued—at the end of the day, we're able to write clear, structured, maintainable code, and delegate much of the mundane or error-prone aspects to Craft or Yii, and focus on the "stuff" that makes that application unique.
Please don't hesitate to join us on Discord if you want to chat about specific capabilities! Best of luck in evaluating platforms.