While you'd be a very brave or silly individual to try to reimplement BLAS yourself, there is definitely value in knowing how to implement any library for yours

While you'd be a very brave or silly individual to try to reimplement BLAS yourself, there is definitely value in knowing how to implement any library for yourself. If programming is behaviour and data, then the deeper and broader your appreciation for data and behaviour in its many forms, the more tools you'll have in your toolkit. Learning by doing is mainly the defacto way of learning for programming, and so while you may never use your own language to ship in production, there is still value in learning to build your own compiler. When you use someone else's framework, you are using their abstractions and their mental model of the world. Lucky for you if it happens to be spot on with your own or aligns with your way of thinking. If the abstraction is too tight though, and the fundamental knowledge is not your own, then when "new-and-shiny" framework is outdated for "newer-and-shinier" framework, your framework-specific knowledge is completely redundant. That's not even starting on confidence that comes with being able to roll up your sleeves and read anyone's code when you've earned enough chops by building various projects.