Actually fiat currencies have value because the government promises to accept them for tax purposes (and for various other purposes -- fees, fines, court settle

Actually fiat currencies have value because the government promises to accept them for tax purposes (and for various other purposes -- fees, fines, court settlements, etc.). The government does not "think" the money is worth something; the people think the law has meaning and is worth following, and so the law gives money its value. The relationship between money and law is a major reason (maybe the only reason) that currencies tend to be neatly partitioned along international borders.