I do it for: fun, learning and planting a flag. Someday you might be looking for a new job and if you have some projects to show off that's the best thing for your job hunt.
(e.g. when I was at rock bottom in my life, my HELOC was maxxed out and I just rolled my car, I started what I expected to be a difficult job hunt so I created an 'application management system' and talking about that landed me a job at an AI company with my interview. Out of the experience writing that I've written a few intelligent assistants such as an RSS reader which shows me just the most interesting articles and an image sorter that lets me easily work with 800,000 images with Chinese metadata even though I don't really know Chinese.)