Do cat ears on cat girls function as real ears? If so, why do some catgirls have human ears too?

Its been bothering me a little recently, and I'd really like some closure or a theory or something. For example the girl in this gif has both car ears and human ears. A lot of anime that have girls with cat ears seem to place a lot of hair where human ears would be to hide it? pls halp.also u fuckin wot?? http://ift.tt/20Au5mf