I don't know about anyone else, but I learn something about myself every time I write a new manuscript. I'm not just talking about the skills and how much my writing has improved, but about the discovery deep in my core, those things I learn about who I am as a person.
Rewind to the first manuscript I ever wrote. That manuscript was me. Just about everything in it related back to me in some way. The main character was me in just about every sense. She looked like me, acted like me, and even had family members like mine. They always say write what you know and oh I did. That said, I never finished that manuscript. And while I learned a lot of what not to do when writing, I didn't learn much about myself, except that I had ideas and I wanted to play with story structure and characters.