Here's another reason to write down your "past states of knowledge and beliefs," this time from the hand of Nobel Prize Daniel Kahneman in his book Thinking, Fast and Slow.
A general limitation of the human mind is its imperfect ability to reconstruct past states of knowledge, or beliefs that have changed. Once you adopt a new view of the world (or of any part of it), you immediately lose much of your ability to recall what you used to believe before your mind changed.
Past events transform your thinking, so the only way to get a hint of how you thought before them is to write down your thoughts before they go away.