Agreed. Writing forces you to shore up the holes in our knowledge and forces you to learn -- that is if you care about what you write. I used to use posting here to add to and clarify my understanding of poker. Writing the book I did had that effect too.
Now I maintain a
blog on a couple of environmental topics. I'm not an expert on those subjects, but learn more with every post. This wouldn't happen if it were just the usual blog providing links to others' articles with a sentence or two of commentary. But by making the effort mostly to write original essays I immerse myself in the material in question and force myself to grapple with it. On the other end, I come out a little more enlightened.