Learn frameworks to invent; learn to invent to understand frameworks.
You can’t understand frameworks without understanding how to write them yourself. You won’t appreciate their design unless you could invent them yourself.
You can’t invent properly without understanding what existing frameworks do. You won’t understand design issues and you won’t be able to design something nice and useful unless you understand what problems need to be solved.
Learn to use Core Data so you understand how to correctly wrap SQLite when you’re writing that financial assistance tool in C. Learn to use Core Data so you know how sweet and easy it can be to animate the UI the next time you’re writing a cross-platform UI library for a game.
What are frameworks? Core Data, Core Animation, AppKit, UIKit. WPF, Windows Forms, various database access layers. GTK, Qt, KDE.