Why was it so important for Caesar to conquer Gallia Narbonensis and Gallia Cisalpina?

It wasn't.

The implication of the question, namely that Caesar Gallia had conquered Narbonensis and Gallia Cisalpina, is simply wrong. These areas have been conquered by the Romans long before, which provided the possibility that Caesar, after his consulate (59 BC), would transfer the administration of these provinces as proconsul. He used this to conquer all Gauls from Gallia Narbonensis (when he took a threat to this province by the Helvetii) and to take warfare to Germania and Britain. The larger part of Gaul newly conquered by Caesar was called "Gallia comata" by the Romans. Under Augustus he was divided into the provinces of Gallia Lugdunensis, Gallia Belgica and Gallia Aquitania, none of which was identical to the still existing province of Gallia Narbonensis (original G. Transalpina).