Urban centers throughout the Silk Road were useful points for merchants to meet, sell and buy goods for transporting to distant places. Thanks to the cities no merchant had to travel all the road, but just sell his/her goods in the nearest city in order to some other merchant take it from there.
The Silk Road was an overland route used to transport goods for commerce. It was, in fact, two routes: one that connected Eastern Mediterranean to Central Asia, and another one that connected Central Asia to China. Silk Road also involved sea routes that, for example, transported goods to Rome through the Mediterranean and to Japan through the Pacific.
Therefore Silk Road was not one route, actually, but a network of routes where cities were clusters connecting various merchants from different places. Merchants would take goods from one city to another and little by little these goods were getting from one point to another. So the cities served for meeting, selling and buying goods in order to transport them farther.
