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

Everyone of us had an opportunity to exchange something with another person. In numerous cases we provide our money to other people and we receive a good or service that purpose is to fulfill our different demands. However, in some cases we also frequently exchange goods in a way that was known some hundreds of years ago, when people tend to offer for example milk for materials to make clothes from.