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 possibility to exchange different products with another person. In a lot of cases we get rid of our money to our partners and we receive a commodity or service that goal is to fulfill our different requirements. On the other side, in some cases we also frequently exchange commodities 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.