Since I try not to assume everyone knows every idiom, there's an old expression to the effect that the only constants in life are death and taxes. The latter is generally a superset of the former, since after all we tax certain forms of death. (That particular pejorative for the very reasonable estate tax, by the way, comes to us care of a Republican propagandist.)
But there seems to be some confusion about why we all carry around these little pieces of paper, and why we're willing to devote large amounts of our time and energy to acquiring more of them. But first let's go through a few common answers that are, nonetheless, probably wrong.
In my estimation, the most common misconception is that money gains value from currency law. For example, the US Constitution only gives the Congress the power to issue money (which it's delegated to the Fed), and the FBI enforces that position. But while it's conceivable that this is a necessary condition, it clearly isn't sufficient: consider the Zimbabwean dollar, which, in spite of currency law, stopped being used when the inflation got too bad. Of course, Zimbabwe demonstrates how this isn't necessary either: the Zimbabwean black market transacted in US dollars, which certainly was not enforced by Zimbabwean currency law.
This moves us cleanly to the "greater fool hypothesis," that I'll accept dollars because I assume you'll accept dollars, and you'll accept dollars because you assume that the grocery store will accept dollars. Partly this is problematic because it's the mother of all bootstrapping problems: why then did the first person accept the first dollar? The other problem is that this would be essentially unique among assets: even gold has more to it than just the Chicago Mercantile Exchange. For all other financial assets, people require something to justify holding them, whether it be coupons, dividends, or the promise of dividends later. So we're either left believing that money is the only financial asset where there are always greater fools, or there's something else going on.
But then, I did telegraph my answer from the start: there is only one thing in the universe that must be paid with US dollars, and that is US taxes.
To see why this matters, let's consider a hypothetical American store that accepts British pounds for purchase of goods. So let's take some reasonable assumptions, that there is no sales tax, but there is income tax. When customers come into the store, they exchange pounds for widgets. (All stores in my examples sell widgets.) The store naturally has employees, who expect to be paid. Since the store has no source of dollars, let's assume they pay wages in pounds. However, when the company pays out wages, it also withholds a portion of the wages to pay income tax.
But now the store has a problem. They only have pounds, but the income tax is due in dollars. The store now has to go to foreign (currency) exchange markets to sell some of their pounds to get dollars to pay the income tax. (If instead the income taxes are paid directly by the employee, i.e., they opt not to have any of their paycheck withheld, then the employee will face the same problem when tax season comes around.) This now opens the company to foreign exchange risk (their tax liability is now dependent on the USD-GBP exchange rate), and they also have to pay forex transaction fees on their taxes.
Now of course there comes a point where it's worth considering the involved financial calculus, as in the case of Zimbabwe, but for country with a stable economy, all these transaction overheads would kill a company trying to transact in a foreign currency or with precious metal or whatever else. It's also worth noting that this doesn't mean that everyone has to be taxed, since for example if there were only a transaction tax that were only charged on stores, they would still demand that their suppliers accept dollars, and that their customers pay them.