What is the reason why people tend to trade and why is this sphere that crucial in modern international economy?

Everyone of us had a chance to exchange different products with another person. In a lot of cases we offer our money to other people and we receive a good or service that purpose is to fulfill our miscellaneous requirements. However, in some cases we also often exchange goods in a manner that was known some hundreds of years ago, when people tend to provide for instance milk for materials to make clothes from.