What is the reason why a lot of customers tend to trade and why is this topic that crucial in modern international economy?

Everyone of us had a possibility to exchange different goods with somebody else. In a lot of cases we provide our money to other people and we receive a good or service that purpose is to fulfill our various needs. However, in some cases we also often exchange goods in a way that was known some hundreds of years ago, when people tend to provide for instance milk for materials to make clothes from.