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 chance to exchange something with another person. In numerous cases we provide our money to other people and we receive a commodity or service that goal is to fulfill our various demands. On the other side, 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 example milk for materials to make clothes from.