They act as a clarification of the sites intent, and any reputable site owner will chose accordingly. Sure anyone can buy any, but if they plan on making a respectable presence on the web, they should keep the meanings of domains in mind. Why does BestBuy's site use the .com domain, because it is a company; why does the Gentoo Foundation use the .org domain, because it is an organization; why does Northeastern University use the .edu domain, because they are an educational institution.