Existing monetary units are subject to changes which have little significance totheir valueas an agent with which to buy goods and services. They are subject to industrial unrest, wars, catastrophies, governmental policies, economic mishaps, and, most spectacularly, to speculation by banks, etc. and the trade in money.
Because of the way that they are backed by goods and services that have already been ordered, the U.N. Dollar is not subject to flux in the same way. Moreover, a separate currency makes it easier to ensure that it can only be used for the goods and services supplied by the Supplementary Economy, and thus help our world to make the transition to sustainable development.