Why the world counts in dollars: a hotel in New Hampshire, 1944.
In July 1944, 44 nations at Bretton Woods agreed on a monetary system: the dollar was pegged to gold at 35 dollars per ounce, and every other currency was pegged to the dollar. The IMF and World Bank were founded alongside. The system made the dollar the world's currency, which it has remained.
Useful: Oil, commodities and much debt trade in dollars because of a 1944 decision.
Source: de.wikipedia.org