Less flippantly, if most major market actors "know" that the US$ is "safe" then that is what they will buy during a panic.
Moreover, if everybody knows that everybody else "knows" that the US$ is "safe" then the US$ really will be safe during a panic, at least compared to everything else. That's an unstable sort of "safe," though, because any time a sufficiently serious actor decides to call the emperor on his nakedness, it stops being safe.
Oh, and then you have the fact that most transcontinental trade is denominated in US$, and it takes time to change denomination on all your trade contracts, so you want to have a war chest in order to avoid being squeezed between short-term contracts with your suppliers and long-term contracts with your customers, if the reference currency goes down (or vice versa if the reference currency goes up, but for the US$ that does not seem a serious prospect...).
- Jake If you only spend 20 minutes of the rest of your life on economics, go spend them here.
