[Edited to make it more understandable question]
What if all nation use only a one currency or one global currency?, What will happen to our world?, What is the effect of every nation's or every country economy?, It can make people's unite each other?Any opinion.

National economy of countries will be badly effected because not all the countries are on the same economy level. There will be no foreign exchange as there will be no other currency, the trade will fall down and so the worth of the global currency. There will be a war between countries for the central banking authority because there will be only a single banking authority which will print and supply the currency.