Look up the petrodollar.

If I'm Romanian and my currency is leu (RON) and you're Mexican and your currency is pesos (MXN), you don't want my RON since you can't use it for anything except for imports from Romania and I don't want your MXN since I can only use it for imports from Mexico.

If we both agree on USD, I can go to any other country which wants USD (all of them) and buy whatever I want.