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 an opportunity to exchange something with somebody else. In majority of cases we get rid of our money to our partners and we receive a product or service that target is to fulfill our diverse needs. However, in some cases we also frequently exchange goods in a way that was known some hundreds of years ago, when people tend to provide inter alia milk for materials to make clothes from.