A single currency is a unified currency which is used by more than one country. The classic example of a single currency today is the euro. Euro is used by all the member states of the European Union. A single currency benefits the economy a lot. The goal of the European Union to follow a single currency was to achieve those benefits which a single currency could offer.
In the early 1990s, an Economic and Monetary Union (EMU) was launched that explained the potential of the Single Market. The decision of establishing a single currency was also seen as a tool for integration and unity. Basically, the idea of a single currency was a tool for fighting back rising nationalism at that time which was not beneficial for the countries. A unified currency in the EU benefited the states mostly in the area of trade. Trade system in EU does not apply any taxes when it comes to trading between the member states. People can easily sell and buy goods and services within all the states of the European Union.
By taking the decision of following a single currency, the EU believed that such huge similarity existing among the states could strengthen the relations and the integration will be irreversible in the future as well.