Rather than the result of thinking, writing is thinking. Translating the ideas of others into one's own words forces deep consideration of what is being said, and thinking through any contradictions, gaps in logic, or elisions in the text.
Writing further provides a productive feedback loop: to be able write something clearly in our own words, we must be able to understand it.
Related
Citations
Ahrens, Sönke. How to Take Smart Notes: One Simple Technique to Boost Writing, Learning and Thinking – for Students, Academics and Nonfiction Book Writers. Sönke Ahrens, 2017.