Here's an idea: (PeterH please note that this is pure speculation - I do not offer any scientific proof)
Perhaps the stone monuments were built to "contain" the sun and moon; to imprison them; to make sure that they never stray outside of their known cycles. Just as men could corral animals in a stockade, they could corral the chaos of the sky within a cage of stone.
It's an interesting observation that the word "lunatic" derives from "lunar".
Imagine: "We've put that big stone over there and the unruly moon can never get beyond it, not even during its wildest wanderings. We are the ones who can tame the chaos of the sky." That's a lot more powerful than saying "That stone aligns with the maximum traverse of the moon."