A. When the world have a single currency it will remove or eliminate exchange rate fluctuations there by making international trade and investment more productive and competent and how can this be achieved it will simply remove the risk and Instabilities connected with fluctuating exchange rates.
B. When the world have a single currency it will reduce the cost in making transactions because there will be no more conversion of currency to another currency and this will really benefit business men and women and also travelers, some time when you travel to another country you fine out that the money with you that you think is big is actually small when converted to that county's currency you travelled to, having one currency will make for easy money transaction between two people in different nations.
C. Having a single currency will also help in enhancing price transparency it will be difficult for a business man to import goods into a country and increase the price x 10 of what he bought it, having a single currency will help to reduce price of some specific items globally.
D. Having a single currency will help to unit the world more and every nation will be his brothers keeper and it there will be a global monetary policy this will help prevent corrupt politicians from stealing.
Even during Anarchy, humans form groups and in the end, the most powerful group rules the country. Humans are also nationalist and racists by nature. It's impossible for the world to have one currency and live in harmony because in the end, the world is too big for us and we are too small for the world to live under one governance, there are seven continents after all.
In order to globalize the world, we have to mix well with each other and kill the racism and nationalism. Is it possible long-term? Definitely but I believe that it will take centuries and there will be blood.