Why would a country such as the United States, which can presumably produce everything it needs itself, choose to trade with other nations?
In general, countries trade with each other because of an economic concept known as comparative advantage. According to the law of comparative advantage, a country should focus on (specialize) in producing and exporting resources which it can produce at the least opportunity cost and import the resources for which it is a high cost-producer.
To further understand this concept, let me explain what opportunity cost means. Opportunity cost can be remembered as what you give up/ what you gain. It is simply the cost of something forgone ...
Gains in trade through comparative advantage. Why nations trade with each other.