Sometimes when a cat grooms another cat, it’s nice. A cat helping another cat out, cleaning those hard-to-get spots on the back of the neck with a friendly lick.
Other times? It’s more like, “Hey, I want that nice nap spot, and I’m going to annoy you until you give it 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.