From Tiago Forte:

You see, I have to write to know what I think. All my ideas sound brilliant in the echo chamber of my own mind. It is only when I put down my thoughts, letting them stand on their own strength, that I start to see the cracks and imperfections

It is almost as though our ideas in our minds are of the lowest fidelity. By writing them down, they become more concrete, and then we are better able to gauge their actual merit.