There were various reasons for the creation of a city in the Roman Empire.  These factors include economic importance and trade as well as strategic value and military outposts.  Cities chosen for economic reasons were centers of trade routes or were in regions with abundant resources.  Other cities were built depending on their military and strategic value.  These cities could be based on their geographical importance in the Empire or for military capabilities.  Outposts were often created in order to maintain the peace amongst regional tribes.  For example, Valencia was an important military stronghold, Tolosa was an economic crossroads, and several military outposts in northern England were founded to keep the peace.  Of course no Roman city served a greater purpose than that of Rome. 

This map shows the extent of the Roman Empire at the time of our Lord, year 150. 

Martinelli, Maurizio.  Ancient Rome: Then and Now.  Florence, Italia: Casa Editrice Bonechi, 1997.