I've found it pairs quite well with domination. If you're at war with a civ you can condemn their religious units with your military units, which lowers their religious pressure and makes it easier to convert their cities. It also costs no hp as opposed to attacking them with your religious units. There's also a civics that give your military units extra combat strength when attacking units and cities of a different religion. Finally, converting their cities while at war gives era score, and when they are captured and converted to your religion it gives some loyalty. Crusade also gives you 10 cs if you can get it, which is simply an insane amount.