My thinking is that it allows for non-rulers to have an item to press for conflict. One of the main depressors of conflict tends to be that rulers have the most to lose by war, but if High-ranking priests in a dominant (say 50% of the pop+) religion can declare an opposing state the target of a crusade, and there is then a penalty for not being at war, but a small benefit for being at war, it can really push sedentary rulers into action.
It also gives the powerful priests a lever over public policy.
It didn't take long for people to reach for the stick, though.
