How does trade help to enrich a civilization?

Multiply reasons. For one thing it allows for commerce, and importation, and exportation, thus allowing new goods to be spread to new areas, thus perhaps improving farming. Trade may also lead to the spread of new ideas or customs, think of how the Hellenic civilizations spread their culture through trade. Trade can also help to enrich diplomatic ties between nations (Or Civilizations), and finally it may lead to exploration.