It's a promise to cancel other liabilities in an equal amount to the denominated face value. Nothing further, yes, but that it functions at all has its root in its ability to make the men with guns leave you alone at tax time. The government agrees to accept these "leave me alone" tickets, and in exchange, the bank agrees to provide the government with them most any time it wants them to buy things.

It's just the grease in the kleptocracy machine.