

Inspire allies to perform better or push their limits, bolster them against mental damage and heal mental damage they’ve sustained, raise the morale of a group (especially if that group shares my religion). Bonuses to public speaking, negotiation, religious knowledge, etc.
If I belong to a more fire-and-brimstone type religion, perhaps I can intimidate sinners, make my allies more effective against heretics, etc.
I guess the video games probably aren’t canon, but I think it was The Force Unleashed that “revealed” that Vader intentionally planted the seeds of the Rebellion to create an opportunity to overthrow Palpatine, which makes all the times he or his troopers “fail” to stop the heroes make more sense.