Without further ado,
Writing is thinking. Writing allows you to process your own thoughts.
Writing is persistent. When you write down what you think, you can save your thoughts and reload them.
Writing gives you structure for tackling more chaotic problems. We can’t store everything in our brain. Yet, we want to tackle certain larger-than-brain problems. Writing helps you do that. With writing, you build a mental scaffolding for yourself. With that scaffolding, you gain power. Your mind clears up, and you gain confidence in your own thinking.
Writing lets you iterate on your own thoughts. You already believe certain beliefs. With writing, you gain introspection into your beliefs, and your history of beliefs. By clarifying your current position on a topic, you can revisit that topic later. You can agree, disagree, or improve that earlier belief.
Writing gives you a deliverable for anything. What do you produce at work? Designs? Software? A service that people use? The thing that you produce at work is your deliverable. That deliverable can be used by someone else. With writing, you can gain a deliverable for anything. Your thoughts can now be shared, and benefit people in the future. Since you now have a deliverable for anything, you can work incrementally on anything. Either extend an existing text directly, or explore next to the text. Then come back to the text and its annotations.
Writing lets you exercise storytelling—the medium for persuasion. We humans understand the world in terms of narratives. A person, a goal, a situation. Actions. A conclusion. I believe that the way to change the world is by presenting and implementing alternative narratives. You can build those narratives through writing. Then you can, should you choose to do so, move that story to other media.
Writing lets you build knowledge. By writing knowledge down and naming pieces of knowledge, you can refer to your knowledge. You can build knowledge on different knowledge. That knowledge is amusingly also easier to remember when you’ve written it down and named it.