It's useful for both, actually. It's basically depth perception for both senses, since we've got 2 ears and eyes. At first, you're hearing/seeing from two points. Then when you tilt your head, you're rotating those two points on a different plane, giving you a different frame of reference.
They tryna hear better. They shift their head to get sound to bounce in their ears differently, allowing to pinpoint location of sound. Good for hunting rodents and looking cute as hell