If all countries in the world use the same currency, let's say the US Dollar, for example, then individual countries will lose the ability to print their own money and successfully conduct their own fiscal and monetary policies.
The main reason why governments have their own currencies and do not just adopt the currencies of their neighbor is because a country's ability to print and create their own money, and thereby control their money supply, is a crucial aspect of fiscal and monetary policy. Thus, if a country gives up their currency for the US Dollar, for example, they are effectively making themselves dependent on their ability to buy US Dollars in the international financial markets for their money supply, which hampers their ability to make their fiscal and monetary policies.
The ability to control a country's money supply is crucial when making sound and independent fiscal and monetary policies. This is because governments need to be able to manipulate their money supply, either by printing more or less of their currency in order to target inflation, ensure price stability, and guarantee medium and long-term interest rates. Sometimes, government will even print more money in order to finance state spending. Thus, if they start using another country's currency, then they will be dependent on how much of that other country's currency is available in international financial markets, which they have no control over. Unlike if they had an independent currency, which they can print at will.
In this respect, as long as individual countries are independent of each other, it will not be beneficial to have a single currency. Perhaps the only time that the world can have is a single currency is when there is a single world government who will make fiscal and monetary policies for everyone. Otherwise, the idea of a single currency for all the world's governments and economies will just place everyone at a disadvantage and dependent on another government.
Source:
Money Creation. https://en.wikipedia.org/wiki/Money_creation
Money creation
Money creation is the process by which the money supply of a country, or of an economic or monetary region, is increased. In most modern economies, most of the money supply is in the form of bank deposits. Central banks monitor the amount of money in the economy by measuring the so-called monetary aggregates.