It's a bit niche. Clergy have a 30% chance to become the new religion head when the current one dies so you can use it to put people in the position that will be highly favorable of you or your heir which helps boost opinion. Clergy themselves contribute some of their own positive opinion to the religion. So for the most part they're opinion boosters. Outside of that, if you have lots of food then pagan clergy can run a mission that's similar to pacify for 200 food in any pagan city. World religion clergy can serve as agents in cities that have the same religion. That's pretty useful for creating agents if you need more of them.
Any clergy member can also tutor kids, so if you're lacking courtiers or they all have jobs, you could create some clergy to tutor, too.