Ok, I'll admit, rogue nation isn't precisely accurate. But by staying outside the international community and maintaining a fixed Currency China risks being labelled a currency manipulator something which we (the world) all want to avoid.
Currency Since the Second World War.
Currency is a complicated topic, and many nations see the strength of their currency (not in value but in prestige sense) as directly tied with their sovereignty. It is not accident that the preeminent power since the Second World War, the United states, has enjoyed the dollar being the de-facto trading currency of choice. The US was able to use the strength of the dollar to push multilateral free trade agreements and in an age of the Cold War push capitalism successfully across the globe.
Since 1973 with the end of the gold tied Bretton Woods system, most western nations have allowed their currencies to float, that is allowed their currencies to be traded on the foreign exchange markets. This is crucial as freely floating currencies promote market efficiency, money flowing in and out of the economy doesn't need to be accounted for by the central government. Furthermore a freely floating currency reduces the need for government intervention in the financial markets (the price of currency doesn't need to be set) which is always a positive.
Having a freely floating currency also reduces the need to physically back your currency, so long as your currency remains stable there is no need for the central bank to provide surety in the form of gold or other valuables.
Admittedly the system isn't perfect, a floating rate can be highly volatile, causing headaches for financial markets as the going rate for a particular currency fluctuates widely, especially in times of crisis.