[dathery]

Knowing you will get paid back (i.e. credit worthiness) is part of it, but another part is that the currency holds its value, and the global demand for dollars is an important part of that. If the global demand for dollars decreases, then borrowing is more expensive because you need to offer a higher interest rate to entice people to purchase bonds, because they have other more attractive options to store/grow their money than US bonds.

One way to think of it is that the US benefits from the current world order by essentially taxing the rest of the world to pay for its spending by devaluing their currencies relative to the dollar.