Unleashing the secret argumentation department of your brain
One of the best things about putting your thoughts in writing is the act of doing it. You're forced to take your thinking to a much more structured level, which often reveals new areas of insight. The process of subjecting loose/rapid thinking into written sentences (instead of the usual speech fragments) and paragraphs requires you to get consistant and invent supportive arguments for your position.
Some of this happens conciouesly, that's the stuff you capture in the first draft, and some of it happens unconciouesly, that's the stuff you get access to when your position is challenged. It's like a secret argument department in your brain is running a covert operation to build surprise defences in case of an attack.
Which of course leads to the notion that it's good to put your thinking to the test. Expose the weaknesses. Have someone take an aggressive stance on your ideas, and you'll be able to leverage the power of your secret argument department. Incorperate the surprise defences you uncovered in another draft of your thinking and the cycle repeats.
I've already started to see the effects of this in our lunch discussions at work. If we're dealing with a subject that I've treated in writing the fragmentation of my oral arguments is significantly less. At first it feels strange, but then you start to think about it, and write about, and all of the sudden it makes perfect sense.