"Coin of the realm" is not an IOU, it is "money". It can be exchanged for goods or services and is a store of value.

In the US today, we no longer have coins, but tokens, since they have little or no intrinsic value. They are not a promise to pay anything other than themselves, just as the US currency is only a promise to pay the face value in US dollars... which only exist as other pieces of paper.

Modern society has drifted a long way from the original fundamental idea of money. Maybe we will return to basics someday after the crash and the monetary system has to be reconstituted.