Every working man or woman in this country exchanges his or her life, one week at a time, for a promise of payment--the paycheck. But nothing is delivered to the owner of the paycheck except other pieces of paper which themselves purport to be a promise of payment. We give all our work for no pay.
"Promises to pay" are "payable" and "legal tender."
The best of both worlds would be for the government to pay its way by issuing IOUs. Yes, that would work for a while, but when the IOUs were presented for payment, what then? That problem could be prevented if 1) the IOUs were never presented for payment, or 2) the IOUs could pay for themselves. And both of these happy circumstances can be brought into being by simply declaring the government IOUs to be "legal tender." Legal tender laws compel you to accept a promise in lieu of payment, while easing the blow by allowing you to spend the promise, or, to put it another way, the issuer of legal tender tells his victims, "Yes, this is counterfeit that I'm giving you for your work, but I've made arrangements for you to pass it without penalty." So there is no point in presenting the IOUs for payment.