Github provides a convenient place for me to publish my work to the world, allowing others to pull it as they desire (my work machine is a laptop, and can never be relied upon to be up, or at any particular address). Because git is distributed my workflow is not effected in the slightest by "somebody else's" (github's) completely separate repo being down.

Contrast this with something older like subversion and sourceforge. If sourceforge went down you were shit out of luck.