MMT time! What makes a currency "have value" is the power of the entity creating it, mainly through the monopoly of violence and taxation. Dollars have value because the US taxes in dollars, and having no dollars to pay the taxes means you go to jail so everyone needs dollars!
Contrary to popular belief, currency didn't spawn from barter and evolving into metallic coins, the first examples of currency were "paper" (technically clay) money, where an accountant at the central temple of the first city-states of Sumer would record the payments and debts in clay tablets. "Katie ows this month the collection of 200 units of grain to the central temple authorities" is the first kind of currency, and people did use these tablets as currency, as in "I will cover your debt if you give me 3 chickens", exchanging this way an obligation for actual goods.
This tells us a lot about the nature of money: money isn't just another good to trade, it's an accounting unit made to record debts!

















