I love Shelling out: The origins of money blog post from Nick Szabo as it's a great explanation of how having a hard monetary system with small inflation shared by everybody the parties to be more peaceful.
US had a trade relation with China where China was buying US bonds, but as US was issuing lots of new bonds China lost trust, and started selling it thereby opting out of the US bonds based monetary system.
Ignorant people used to joke that China owned the US because of their treasury position (despite the fact that Japan's holdings were roughly as large). Well that position has been massively diluted over the past decade and is now largely meaningless. The Fed can eat China's position with a snap of its finger.
It's to China's benefit to hold the position, at present.
It's not very productive in the long run, however - for example, a tank is only 'economically productive' if your economic model is based largely on raiding other country's resources. A tractor, or construction crane, however gives added economic value because it can be used to improve agricultural production, build better infrastructure to facilitate trade, etc.