Personified concepts realized by mortals don't need planes. Or underlings.

But the Prince IS the plane. The Avatar isn't the Prince. It's just a method to communicate with a mortal in a way they understand. And "underlings" are there to simply spread about the Prince's desires. Werewolves are a great example. Werewolves are there to be a great embodiment of the hunt. They hunt by night, and are hunted by day. That's what they're for. The Princes enjoy spreading their spheres.

I don't think game mechanics play a part in this one. All Daedra are just generic enemies. I think, for example, Golden Saints would go around causing people to go insane. Perhaps they emit an aura of insanity? Spending too much time around one makes you insane?