Quote Originally Posted by Sarcasm
Think of Massilia as the interface between the Mediterranean world and the continental (as in Gallic) world. There's massive amounts of trade going through it, and in order to reach the rest of gaul the trade route must cross Arverni lands. Taxing and downright raiding is an awesome way to get wealth in order to fight your wars.
Cunliffe's Greeks, Romans and Barbarians has a lot on this topic.