All through school, I knew there were classes and there were tests and exams. The classes were where we turned up to hear things for the first time, internalise and understand them, while tests and exams were where we went prepared to prove what we had understood.

However, going prepared to the classes would have made the in-class experience more efficient and productive. 

Outside of the school setup, where there are no distinctions as to what is a class and what is a test or an exam, the mental model of being prepared for one kind while not the other no longer works. 

When in doubt, it is better to be prepared.