A cornerstone of America's international economic might is the dollar's status as the international reserve currency. Simply put, the dollar is the device that parties use for international transactions — even those that do not include the United States. The readiness to use the dollar reflects the parties' faith and confidence in the stability of that currency. They believe its value is predictable; thus there is more certainty in the exchange.
The country whose currency is held in reserve enjoys immense economic advantages. It does not have to worry about exchange-rate fluctuations having a direct effect on the trade of goods priced in that currency. Americans can be certain of the price of a barrel of oil priced in dollars; Japanese (or any other purchaser who does not use dollars) are not. They must accept the costs in dollars and then calculate the price in their own currency, and that exchange rate changes daily.
Alternatively, those other countries can purchase and hold dollars, effectively lending the U.S. their savings. Today, about two-thirds of global central bank foreign exchange reserves are held as dollars. This lowers interest rates in the U.S., providing a real economic advantage for the U.S. If the currency is stable or expected to be stable, such lending makes sense. If a devaluation looms on the horizon, it does not.