Because each country has resources and production capabilities are not equal. Make each country needs to import and export goods in order to be able to bring those products to meet the needs of the people in the country.
Because each country has the resources and capabilities to produce unequal. Allows each country to import and export goods to be used in those products to meet the needs of the country.
Because each country has the resources and ability to produce symmetrical. Make each country must have import and export goods to the merchandise to meet the needs of people in the country.