Writing is thinking, distilled.
Everyone has ideas that hit every other second. And ideas are a dime a dozen. But when those ideas are written down, gaps in thinking are exposed and as the idea through emerges on paper, tough questions stare at the writer. There is no running away from it.
A good writer use the many tools of the craft to can fool the world, but not for long. More importantly still, you cannot fool yourself. That is also why, writing is pure meditation as well.