The (exchange) value of anything depends on supply and demand. Demand for USD comes from many places, but most of those are based on circular reasoning: Employees want to be paid in USD because they need USD for their shopping, the shops want to be paid in USD because they need USD to pay their employees.
Taxations is the only entirely non-circular source of demand.
(Bank loans are a secondary somewhat non-circular source of demand for USD, and that explains why bank loans can increase the amount of money in circulation without increasing inflation.)