My best (stupid) guess is weapons. US gov has a ton of weapons. Already sells tons of weapons. Now the prices will always remain stable. Other countries would love to know that 30 million usd would always be able to buy an f15. Other countries declaring war will increase the value of the USD, as buying weapons from us government will decrease amount of money in circulation.
Because controlled inflation is necessary for an economy to function and nothing but a fiat currency has the desired characteristics.
Gold is still the best option if you insist on having the currency backed.