Conceptually, open source is important because it means sharing knowledge. Since we are all [supposed to be] developing technology because it makes the world a better place, then we should be open in sharing how we are coming up with these improvements. Pragmatically, it's important because the world is adopting technology, and without this tech being open, we would live in a world ruled by technology that we can never hope to understand.
I consider myself an open source enthusiast, but that's probably understating the impact that open source has had on my life. I learnt very little from closed source applications and systems, and profit-driven schools and courses. By contrast, I learned most of what I now know because of open source, the hacker culture, open ideas, creative commons, and friends who openly share their knowledge.
I want to help promote that, and protect it.
In addition to caring about open source, I also contribute to it. I'm not a developer by training, but I do work on a few small projects; I maintain several packages for Slackware Linux and Slax Linux, I maintain the current version of Emacs's Screenwriter-mode, and I have written various tools and utilities made available through Gitorious.org.
Also, I teach. I have written articles about open source software for magazines like Linux Journal, Linux Format, Linux Identity, opensource.com, I wrote the new user manual for Qtractor, I have written an introductory book about learning Python, and I post tutorial videos and podcasts on multimedia content creation (which is pertinent to my, for which I use open source).