You'll ultimately become a better dev (e.g. higher quality code, faster, and more maintainable) as you learn more of the underlying stuff. For web dev definitely learn how to do everything in "vanilla" js, html, and css at least a few times.

Some people think better at a lower level and some think better with abstractions. I for one feel more 'at home' at the lower level but can live with higher level abstractions if I know how it's done at a lower level :)

Someone needed to build the framework. Depends what your goal is.