Debt is slavery. It's simple as that. It's a tool that can be used against individuals on one end of the scale (credit card usury, predatory lending, etc.) to resource-rich but under-developed countries on the other. Historically, third world countries have been manipulated into massive debt schemes to the benefit of oil companies and the overall US corporatocracy.
John Perkins has an excellent primer on global oil debt from the addendum of the Zeitgeist movie. We all need to think about the true costs next time we fill up that tank.