“I didn’t know I needed that” is a common expression of surprise and realization. It suggests that something, whether it’s a piece of information, a lesson learned, or a newfound understanding, has come to light and is now recognized as being essential or valuable, even if it wasn’t previously apparent.
God knows what I need even when I don’t have any idea. The Holy Spirit makes it clear, week after week, that if I lack Christ, I have nothing. But if I have Christ, I have exactly what I need.