Specifically it’s the requirement that taxes be paid in the local fiat currency, with the police there to back it up. The fact that you are obligated to get you

Specifically it’s the requirement that taxes be paid in the local fiat currency, with the police there to back it up. The fact that you are obligated to get your hand on USD to pay taxes has an effect of tying USD to the real economy, giving it value. For USD it’s also the fact that oil is sold in USD, which is why a lot of foreign debt is also denominated in USD.