An idea can feel clear in your head and become less convincing once you try to put it into sentences.
Writing is a useful way to find out what I actually think. In my head, an argument can feel complete because I move past the gaps without noticing them. On the page, the missing connection is harder to ignore.
That makes writing more than a way to communicate a finished thought. It is part of the process of working the thought out. Some ideas become sharper; others turn out to be less interesting than they first seemed. The page gives me something concrete to question, revise, or leave behind.