This is a great question. Trade is one of the most important ways in which an economy grows. Moreover, tied to this fact is the growth of cities. Let me give you a few reason why this takes place.
First, with trade, there is a building of infrastructure. Think about trade in the High Middle Ages. You would need roads, supplies along the way, outposts, money changers, buyers, sellers, and so much more. All of these allied things and people are tied to trade. When this happens cities experience growth.
Second, port cities are particularly prosperous. Until the invention of the steam engine, ships were the fastest way to transporting anything. This is why port cities grew rapidly.
Third, when trade grew, more and more people left the life of agriculture. There were two reasons for this. First, you needed people to do the actual trading and traveling. Second, when there was trade, you needed people to support them through labor. All of these fact go into the growth of cities.