Fiat does have value for three reasons:

1. If you are a creditor and refuse to accept fiat as repayment of debt, the state monopoly of violence will not help you collect your debt.

2. All entities that have accounting obligation by law are required to do their accounting with the official fiat current as the unit of account.

3. All taxes and fines must be paid in fiat.

Those three things make using the official fiat currency the path of least resistance. Because everybody else accepts fiat, people generally accept it. That, however, does not mean fiat currency is impossible to debase beyond usability. The world has seen plenty of examples of that. In our time, the government has ruined fiat in Zimbabwe and Venezuela by hyperinflating it.