Grew up with German Shepherds that all did this. I actually find it pretty obvious that its about cognition. When they tilt their heads I can just about hear their brain working to figure out what you mean + want. They've got excellent hearing and they don't really need to improve it, they're just thinking about whatever stimulus they're reacting to.

And probably did evolve to communicate that thinking pose to humans. Its a little bit of an "I've almost got it, can I get another hint?" kind of pose.