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.
I mean we would use gold in electronics a lot more if it wasn’t considered valuable. So it’s not perfect because it actually has a wonderful use outside of being valuable.
Rather unlucky society picked gold and silver to he valuable with how useful they.