What is the reason why people tend to trade and why is this sphere 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 other people and we receive a product or service that goal is to fulfill our various demands. Nonetheless, in some cases we also frequently exchange commodities in a form that was known some hundreds of years ago, when people tend to provide for example milk for materials to make clothes from.