One benefit of learning a framework is that you can internalise the mindset of the authors, and another is that you can see abstract patterns and ideas in their native habitat.

Learning Rails taught me about metaprogramming, reversible database migrations, ACID, and the pros and cons of ORMs. Learning how to build XAML apps with C# taught me about two-way data binding, MVVM, DSLs, and socket communications. Learning React and Redux taught me about cooperative threading, functional programming, transactional state management, and testing front end features without using selenium and webdriver.

Now, I already "knew" most of these things, but I hadn't applied them in the real world before. Getting to experience them as implemented in well-designed frameworks taught me a lot about their practical use, building on top of the theoretical knowledge I'd already acquired.

EDIT: cooperative threading, not cooperative multitasking