Survival.
Making sure the enemy did not regard you as an easy target.
As I said prior, fighting them over there so you do not have to fight them in your back yard.
They had no interest in governing the barbarians over the hill, they just wanted them to leave them alone.
People do not build walls because they like not being able to see out, they build them because it is necessary.
hmmm... so you subscribe to the theory of the Aryanic people warring with the local Dravidian/Harappans?