It's simpler than this. Explaining a question/issue to someone involves going back to basics and covering all the foundational info that a third party would not have. When thinking alone, you gloss over this, and may not realize that a foundational assumption is incorrect. When you are forced to explain every step explicitly, these errors or gaps can become apparent without any intervention by the listener.

I have my younger kid explain each math problem to me before she submits it on Khan Academy. My older kid thinks in her head how she would explain a problem before turning in a test. It's a good habit to form.