Writing lets me do that while also helping me avoid going around in circles. When thoughts are in my head, it's easy for them to get jumbled up. I miss things, and I keep coming back to the same thoughts, leading to the unproductive ruminating.

Because text is ultimately a self-referential structure of claims, definitions and facts. When you are writing a new sentence, you have access to the complete accumulation of your previous thoughts in former sentences and paragraphs. The new sentence builds on and extends these connections, and doesn't stray in different directions like verbal speech.