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 something with somebody else. In a lot of cases we offer our money to our partners and we receive a product or service that aim is to fulfill our miscellaneous requirements. Nonetheless, in some cases we also regularly exchange products in a manner that was known some hundreds of years ago, when people tend to provide for example milk for materials to make clothes from.