Non-US economies need US dollars, partly because the dollar is so dominant in global trade. (It’s hard to spend South African rand in Chile or Chilean pesos in South Africa, so typically two countries that don’t have widely used currencies will denominate their trade in some other currency, most often the dollar.) This means that foreign banking systems need dollar reserves, and their central banks need dollar-denominated assets, too. All of this can only be in equilibrium if the US is a net exporter of dollars—that is, if there’s a persistent trade deficit.
