From BL Liddel Hart's Scipio Africanus, you get a picture of why Scipio chose New Carthage as the place to start operations against the Carthaginians in Spain.
Scipio was greatly out-manned in Spain, so he choose a symbolic and logically important place with the campaign - Cartagena, or "New Carthage."
The Carthaginians were confident it was well-defended, since there were four armies within a week's march from there, but Scipio managed to take the city in a few days, which shocked Carthage and put their people off balance - and most importantly, made Carthage's Spanish allies question their support.
From the book -
In summing up this first brilliant exploit in command, the first tribute is due to the strategic vision and judgment shown in the choice of Cartagena as his objective. Those who exalt the main armed forces of the enemy as the primary objective are apt to lose sight of the fact that the destruction of these is only a means to the end, which is the subjugation of the hostile will. In many cases this means is essential -- the only safe one, in fact; but in other cases the opportunity for a direct and secure blow at the enemy's base may offer itself, and of its possibility and value this master-stroke of Scipio's is an example, which deserves the reflection of modern students of war.