I've never been one to do much writing about writing, mostly because I don't know the first thing about writing. My formal education on the subject consists of high school English and a few freshman-level college courses, while my informal education consists of reading a fair bit. The only writing I've ever done outside of school has been on message boards and on this blog. However, there is one significant thing that the act of writing consistently has taught me, and it is just how integral writing is to the process of learning, organizing and understanding.
While the writing process can actually inspire new thoughts and ideas, or illuminate something for yourself that you hadn't been able to grasp before, that isn't primarily what I'm talking about. What I'm addressing is the way that writing forces you to really intimately understand something -- not necessarily some entirely new idea, but some thought or idea you may have always had but never attempted to communicate fully -- in order to attempt to express it in a way that will be cogent, compelling and convincing to readers.
While you may understand some idea in your own mind, if you've never attempted to give the idea written form there's a tendency for these ideas to be loosely understood or poorly anchored to fact or otherwise sloppily constructed. In the process of writing you may discover contours, dimensions and idiosyncrasies in your thought that you weren't even fully aware of. You may locate weaknesses in your beliefs or ideals that you didn't know were there, and be forced to adjust them accordingly. You may stumble upon a felicitous expression that fills out a thought or makes it more robust.
What I've found, over and over again, is that once I've written on a subject, my thinking on that subject is clarified and given a solid structure it didn't have before. Once this happens, it becomes much easier to express myself in conversation on a particular matter. Because I've already gone through the process of formulating my private thoughts into something that is intended for someone else, it now becomes much easier to do so again when conversing. We humans have a tendency to get lost inside of ourselves, and writing for an audience forces us to externalize in a way that we know will be open to evaluation.
You might say that simply socializing with people can do the same thing, and that is true in a sense, but the rigor that is required when formulating entire thoughts into written form is never present in the same way in conversation.
As with so many things, I didn't come to appreciate the lesson I was being taught when being forced to do book reports until much later. While it's possible to read something, never write about it, and largely comprehend what you've read, the process of interaction with the text by way of writing is a more concrete, reliable way to train ourselves to think clearly. My teachers knew that writing is understanding, and now I know what they were getting at.