If you can't explain an idea clearly, you don't fully understand the idea.
This is the principle behind The Feynman Technique.
Physicist and Nobel Prize winner Richard Feynman once said that he could only determine whether he understood something if he could give an introductory lecture on it.
Reading with a pen in your hand is the small-scale equivalent of a lecture. Permanent notes, too, are directed towards an audience ignorant of the thoughts behind the text and unaware of the original context, only equipped with a general knowledge of the field.
Aside from explaining, writing is a great way to think through ideas.
You have to externalise your ideas, you have to write.
Richard Feynman stresses it as much as Benjamin Franklin.
If we write, it is more likely that we understand what we read, remember what we learn and that our thoughts make sense. And if we have to write anyway, why not use our writing to build up the resources for our future publications?
Anne-Laure Le Cunff in Writing as a thinking tool
“Ce qui se conçoit bien, s’énonce clairement,” once said Boileau (1636-1711), a French writer. It could be translated to: “[[E: What is clearly thought out is clearly expressed]].” This is the principle behind the Feynman Technique, named after Richard Feynman (1918–1988), a Nobel-prize winning physicist who has been dubbed The Great Explainer. ([[Bill Gates]] called him “the greatest teacher I never had.”)
David Perell in Why You Should Write Online
An empty white page is a mirror into your mind. When the ideas in your mind are clouded, so are the words on the page in front of you. Re-writing is re-thinking. It’s the best single best way to sharpen your ideas. And once your ideas are as clear as a Neiman Marcus mirror, you’ll be able to teach them to others.