I see what you mean - I suppose it could just exist as a potential punishment for the player, more than an incentive for the AI?

If not, then maybe to counterbalance just raiding an AI capital and taking (part of) their treasury, the AI would become super aggressive against whoever did that? I.e. they would use all their remaining resources to wipe you out, meaning that you shouldn't go behind enemy lines and sack the Seleukid capital, say, without carefully considering the consequences first.