The point of brattiness (in that context and imo) is to "misbehave" in order to provoke a response. For a bratty sub, there's a feeling of "getting what's coming to them," whereas with a bratty dom it's more about the denied gratification and the power of being getting away with stuff when they "deserve" to be punished. A bratty dom can also be a pretty natural setup to "turning the tables" in a scene, playing at exaggerated overconfidence/smugness that leads into comeuppance. Not for everyone ofc but that's the basic idea.