This was very confusing to me - I drink bottled or canned beer; but I think they’re referring to draft beer:
http://www.draft-beer-made-easy.com/carbonation.html
That (your link) was supposed to be easy?
Pressurized carbon dioxide is used to move the beer from the keg to the tap. Running out of CO2 is as bad as running out of beer.
Yea, CO2 is used to pressurize the kegs.