In the instance people were to move or travel out of their country, it would take a burden off many not having to find the trade rate, and get there money transferred example USD ➡️ $C. It would eliminate this problem and make traveling or moving international easier for the traveler.
It won't work because the richer countries will just get richer and the poorer will just get poorer. Eventually this will lead to a communist world. And we know how that will end up. Another big reason is because the stock market will crumble. How will the price be decided?
If all countries adopted a global currency, they would also lose control of their monetary policy. Greece is a good example of what can happen when a country surrenders monetary control. Had Greece kept its own currency, it could have printed more money during the economic collapse. This would devalue the currency, but also increase the amount of money in the economy and hopefully stimulate spending. But because the other countries in the Eurozone did not want to devalue the Euro, they refused to print that money. So Greece had massive cash shortages which probably made the economic situation even worse.
The global economy is too variable, with too many weak or collapsing economies to try adopting a single currency right now.