Leeching techs better. In war, you and the AI got a war value. Each war success increases your value. Other factors like soldier count (powers) enters in the formula and that is where war success favors the human player. With AggAI, if you endanger an AI city (just one is sufficient) with a sole unit, their war value is diminished to a third that it was.
Thus, if you got x value and the AI x value too AND you endanger their city, thus you got a remainder of 2x/3, which can be extorted in form of gold, techs or whatever you want.