Warbreaker was written basically "open source." He released each draft of each chapter as he wrote them, and had a forum set up for direct feedback from fans. This was back when he had a much smaller fanbase and such an approach was possible.
Each revision is still available on his website along with the Creative Commons publishing terms.
As for the why, he explains in this blog post from 2007 that the biggest roadblock for an early author is exposure. Lowering bar to acquire the book means more publicity, and more likelihood that people will pick up his other books. Also he just loves the process of writing and sharing that process.