an evolutionary function. Anybody else?

An "adaptation" need not confer an advantage; it need only confer no disadvantage. Perhaps adaptation is not the best word for this type of change.

Or, if there be an advantage, it has nothing to do with celestial objects, but is merely an adjunct to the very real advantage of knowing how close that bird of prey was to our long ago ancestors. I believe this was discussed in one of the links. Things directly overhead are generally closer than things on the horizon.