What other gear do you have on her? There's a lot of stuff that triggers on radiant damage/illumination, especially that nice armor piece in the Underdark. Clerics deal a lot of radiant so you can really stack the radiant orb debuff with the right items, even just in Act 1. There's a particular legendary weapon that is really good to give her, too.

Clerics don't really do anything as flashy as Action Surging for four attacks or casting fireball, but they're pretty tanky for casters, have good support abilities, and spirit guardians is good sustained damage. They really shine in Act 2 especially.

What level are you? Cleric really shines once you get spirit guardians at 5th level, especially in Act 2.

Trying to heal more than the enemies deal in damage is very rough, though there are some good items in this game that make it more viable. But casting heals with your main action is still usually a waste.

If she's missing a lot, make sure to check hit chance before you attack. She's proficient in simple weapons at least so give her decent weapons and check if they have a better hit chance than her spells.

I used it to press the button to save Mayrina. That's about the only thing that it was really useful for in that whole playthrough, though.