Normally, when you get motivated about contributing to an open-source project, or even get the courage to roll your own, the reasons traditionally announced are that open-source is fun and contributing back to the community is part of what the good guys should do.
While this is true most of the times, it's not the sole reason to roll your own open-source project, there is a very important shift happened in the past few years regarding how employers seek talented engineers, around 15 years ago, almost everything was about the "Certification", you get your "MCP, or RHCE" and you have secured a job, probably after passing a few silly interviews. Today's world is totally different. I -as many others- see certification as a total crap! Technology certificates doesn't mean anything to me anymore, except in rare cases. It's like that showing your driving license doesn't prove that you are a Schumacher. Show me a test-drive and I'll judge if you are or not.
In today's world, this is the code you wrote, you are what you code. Writing good code makes you a good software engineer, how can I see your code unless you are hired?, you got it, Open Source stuff!
I can see your open source code, test cases, documentation, thinking style, even the people who influence you, everything! I can see if your code attracted attention or not, if you know software design or you are just piling your code off.
You know what I want you to do right now, don't you? Go fork an open-source project or let me see an idea of yours (even if extremely simple) on github.com or bitbucket.org
Yeah, it's true. GitHub/Bitbucket is your new CV.