If yes, can you provide an example? Is it due to learning new tricks that you wouldn't learn otherwise by reading other's code? Or any other reasons as well?
Often, I participate in the development of open-source projects I use at work -- there's no better way to maintain expertise on something than to be one of the people who wrote it!
Coding on things unrelated to work can help one avoid tunnel-vision in programming. By staying active in different kinds of development projects, I've avoided "well...we always did it like that" syndrome.
Working in open source, I've had the opportunity to receive guidance from coders far more experienced than myself; this has made me a better coder.
Working in open source, I got real-world experience managing development teams, so I was ready when I came to a leadership position at work.
Working in open source, I get exposed to new technologies earlier than I would in the workplace: when the time comes to evaluate technology Foo for use at work, I have opinions and the facts and experience to back them up.
It's just plain fun. :)
Yes, working on any kind of projects outside of work help expand your knowledge on a particular subject, allow you to network with other developers, and help you build your portfolio.
As a developer, there is no reason not to have something working on the side, whether it is an open source or commercial project or even a technical blog.
Also, I've found side projects a way to stay passionate about programming. On your own projects you get to do whatever you want which can be exciting and a nice change from the day-to-day corporate programmer life.
The more things you work on, the more you will learn and the more marketable you become.
Well... not especially open source, but working for some hobby project in your free time can really help you with your coding skill and with your cv as well.
As example I started using python for a report generating tool in 2001 and then I was able to apply for a work in python in 2003.
It happened again with Java technologies and now I'm learning Scala...