Whenever a new rule(r) is established, that ruler's money is introduced. You can always tell who is really in charge temporally according to the money in circulation. What is the world's reserve currency? That tells you who is the physical boss. Yet, it may not be who you think. You nor I sign that piece of paper which we pass between each other for goods and services. The entity owning that currency is printed across the center top of that paper. That paper is a contract; an agreement between two parties. There's a date, terms and signatures of two party representatives.
As currencies and mediums of exchange (what money is) have changed over time, so have terms regarding their exchange. The rule of law commands obedience to the always changing terms. Why would people forgo trading their local currency, or simply bartering with a handshake and mutual understanding, for a new money? Force. Legal liability. The fact that “taxes” can only be “paid” in that new currency, not in rice, nor chickens nor the previous currency. One man over another by force.
What is the currency used in your household? Is it the empire's paper? Do you have your own? My wife and I have a credit/debt system of our own. It is based/backed by Love, not gold or the issuance of interest bearing debt. We don't trade with interest. Our currency is duty, interest and tax free. We don't keep a strict record of our debts towards one another; we're poor accountants. But we are traditionally crediting one another constantly. And when I mess up, say or do something off the Way of goodness, she forgives my debt. I do the same for her.
We've grown to be the wealthiest people we know by issuing the currency of Love.