Often the military need for such a stronghold had more to do with the foundation of a 'colonia' than the presence of too many mouths in the city.
Cicero, speaking of a 'colonia' planted at Narbo (now Narbonne) in southern Gaul about 118 B.C., and planted perhaps with some regard to an actual overflow of population in contemporary Rome, calls it nevertheless 'a colonia of Roman citizens, a watch-tower of the Roman people, a bulwark against the wild tribes of Gaul'.

Those words state very clearly the main object of many such foundations under Republic and Empire alike.
Another reason for the establishment of 'coloniae' may be found in the history of the dying Republic and nascent Empire.

During the civil wars of Sulla, of Caesar and of Octavian, huge armies were brought into the field by the rival military chiefs.

As each conflict ended, huge masses of soldiery had to be discharged almost at once.

For the sake of future peace it was imperative that these men should be quickly settled in some form of civic life in which they would abide.
The form chosen was the familiar form of the 'colonia'.