It discourages sloppiness. I redesigned bryanbraun.com last week and I also posted the Drupal theme publicly. I've been wanting to do that for a while, but it took some some serious code clean-up for me to be comfortable releasing it. Now it will be a lot easier for me to update and maintain. It's certainly not perfect but it's a whole lot better (...aaaand responsive! Try it on your phone).
You get to practice explaining things, giving demonstrations, even doing some marketing to some degree. All are great skills you should develop.
It acts as a portfolio, which a clear picture to future employers and partners of the kinds of things that you like to do.