You have to bear in mind these marks are very, very old and a lot of them bear signs of erosion which, from photos, makes them appear to be natural but they are not.

As to what they are for, no one really knows. I suspect it has something to do with things celestial but since they come in different formats and configurations they probably had several different purposes.

And I'm not surprised at all that ancient peoples knew a lot about the heavens, what is surprising is the tremendous amount of work it took to contruct huge stone observatories, many of which still stand today and, excepting a small amount of drift, still function. And we know next to nothing about how they did it or who the hell they even were.