When I write something down it somehow feels more real. It's not just a thought anymore. It becomes something actual.
It's easy to think that everything in my head is clear. But often I realize it's not even clear to me. Usually when I try to explain it to somebody, or write it down.
Thinking is just the first part. You need to get it out before you can really take an action. After it's out it's easier to act on it, even for others.
Sure the final part doesn't come automatically by writing everything down, but it can function as a motivator too if somebody else knows your intentions.
When it's written down you don't have to worry about forgetting it anymore. It's documented, easy to check how it was.
I have written many things already. Many ideas that have been in my head for a long time, but for which I haven't done anything. After writing them down I have also acted by those thoughts, made the things I had just dreamt of.