The fact that dollars are widely used in international markets, e.g. in oil trading, is what enables the US to borrow money abroad in its own currency. Which is a pretty big deal, since owing money in currency you don't control (which of course is what most nations have to do) leaves you vulnerable if there is a downturn in your economy and your currency weakens. Keeping the status of the dollar as a major reserve and trade currency is important to the USA.