Open-source matters because it's free to play. Anybody anywhere in the world can open up a browser and get access to the best thinking / practices of projects in esoteric languages all the way through common place languages.

For people who open source systems/ideas/etc (think DHH and ruby on rails for the current hotness, they get thousands of people to play with their framework and make it better. This benefits DHH and colleagues while benefiting everybody else).

For those who contribute, it allows them to show off their knowledge and/or skills. By contributing you get to meet and chat with all kinds of intelligent people as well as learn best practices.