1. You learn how to be efficient (if not necessarily clean) with your code
2. You will network by default in order to solve the problems you encounter - and set yourself up with the network you need to find a job
3. You prove that you have the range of skills needed for a good developer (requirements management, holistic understanding, deployment environment, technical depth)
Could you elaborate on the "network by default" point? I've been programming for 18 years and my professional network is pathetic. Any problems I have I research online - my network is google search. Unfortunately decent jobs are much harder find through google search.
I'm sure you weren't googling answers when you first started and were probably asking your questions on bbs. I would also imagine you are using SO and other "forums" like that which are a form of networking (look as the profiles of some of the top people answering questions). I consider having a public GitHub repo networking because you can invite and share your projects so people can evaluate your work and collab. To me these are the default things that good devs are already doing.
Beyond that though I would suggest attending meetups in your area for all kinds of different things - for example there is a Ruby meetup in our area, there are node meetups, meetups for ML and NLP etc... These are cool places to show off your work, get help on projects and meet other people who would have links into jobs.
1. You learn how to be efficient (if not necessarily clean) with your code
2. You will network by default in order to solve the problems you encounter - and set yourself up with the network you need to find a job
3. You prove that you have the range of skills needed for a good developer (requirements management, holistic understanding, deployment environment, technical depth)