That's an ongoing debate. Nothing conclusive has been determined yet. The only thing certain is they have nothing to do with the actual ears, as you alluded. For those that don't know, owl ears are down near the eyes. There are 2 leading theories for their purpose.
1 - Camouflage, to break up the outline of the bird. I'm not so sold on this. Lots of owls have no plumicorns, and they're usually pretty small if they do have them. Does one of these guys look any better guys to you? Keep in mind that Grey Owl would be much bigger than the Screech Owl. Two little poofs on the Grey wouldn't make much difference, and it's well hidden as is.
2 - Expression, to display mood or to be flirty. This to me seems more likely. Which of us isn't fascinated by them, and we're not even birds! They can either look cute, or it can add to that intimidating death stare. I've seen them compared to our eyebrows. They add that flash of danger or sexuality, like with human plumicorn owner Eugene Levy. π€£