In evolution things usually persist for multiple reasons. It seems possible that the gesture is both functional (in terms of the dog's hearing) and attractive (to humans). An attempt to perceive correctly and a signal of openness and willingness to do so.
Sure. I just think it would be more interesting to know why the authors of the study, who have probably invested much more time thinking about this than any of the commenters here, concluded that that explanation was not satisfactory.
Maybe because they had already decided to stick a particular bread of dog. Border collies are far from representative of the species. They are built for a very particular task, one that is far from universal. There are many things for which they are not suited.
