@edgarblythe,
"I know that", because sometimes crystallizing knowledge is more important than merely knowing something. Many times upon relearning something I better appreciate its importance, or learn a new way with which to apply some of the knowledge.
But due to my impatience, especially in my youth, I have tended to not want to hear things again that I think I already know, and looking back at conversations over the decades I can recall old conversations that had knowledge to impart to me, but that I only fully crystallized years later upon revisiting the subject.
"I already know that" has been something that too often has prevented me from learning something more deeply or in more useful applications.