Money is a useful medium of exchange that supports a reciprocal trade between all parties. Imagine a time when people paid with stones, pearls, salt or whatever was supposed to be rare at the time. Some people had always access to it and it was hard to govern how much money is circulating. It can be argued that money is still not the fairest paying method but we are somehow mostly used to coins and banknotes, so that it is simply more convenient.
Even though there are so many different currencies like euro, dollar, pound, yen and yuan, they all serve the same function and facilitate the trading between actors, even across borders.