Why would somebody do this? Isn’t it already enough to code at work for eight hours a day, five days a week? If you ask yourself this questions, then I think you should reconsider your position.
There is a fundamental difference between work and free time. You are not constrained. You don’t have to hold a deadline. Software development is a mentally challenging task, and while some time pressure keeps you focused, a little bit more forces you to cut corners instead of considering better alternatives. If deadlines were good, they wouldn’t have “dead” in their name. In your free time you decide when you are done.
Even having considered alternatives you are not always able to implement them. There may be a corporate identity that doesn’t contain your favourite flavour of pink. There can be a module licensed under a non-commerial-only license. Or maybe your company uses an old framework missing the latest features. No such problems in your free time.
There is a theory that mastery comes from practice. By coding in your free time, you can decide whether you invest your time in deeper knowledge of some topic or in a broader horizon thus becoming a valuable employee. And sharing freshly won knowlege and experience can increase your reputation as colleague too.
The social among us even meet like-minded people at events like Java User Group or Schneide Dev Brunch. Here the amount of transported information is much higher, since everyone has another background and focuses on another things. You can even share your mistakes and laugh with others about them.
Are there any side-effects of free time coding besides those listed before? Yes. Your personality can change. It is possible that you will gain a positive attitude and start invest your free time in your skills. Maybe you’ll even start to motivate others to do likewise.