I have two females, 14 and 4, and the older female tried to passive-aggressively groom the younger one into moving out of the spot she wanted but the younger one was like 'jokes on you, I'm into this' and now she constantly bugs the older one to groom her face. This has gone on so long they defaulted to being best friends, even if the old lady is a bit of a curmudgeon.

They also do it to assert dominance, which is hilarious to me

I’m glad this serves as explanation for the non cat people and vindication for the cat-havers.

What I didn’t see mentioned was grooming as a means to establish hierarchy. I’ve noticed mine have a sort of pecking order based on who grooms who. Like my momma cat will groom both her babies, and then one of those babies (grown now) grooms his sister and she’s kind of the runt/shy kitty. My other girl is not fond of being groomed by any of them as she’s much more independent.

My cats exclusively bathe each other to initiate kitty cat combat. "I'm bored, do you want to play?" I tried it on my wife and it initiated combat in that context too.

Cats also do it to infantalize each other. "you are a filthy kitten and I'm doing this because you're my little bitch" which cats also play the "I'm just a cute wittle meow meow pls help" card so its never clear which or how they are manipulating each other than the complexity and range of their interactions.

I have a cat that grooms passive-aggressively. If she doesn't think the other cats are up to her very specific grooming standards, she'll groom them until they are, (whether they like it or not).

Sometimes?