The fact that they're 20-50 meters in diameter could mean dwellings of some sort. And larger kite-based structures something of religious significance.
Otherwise, they could have indicated a route. I remember when I was hiking in Chile, finding any sort of stone arrangement that indicated some other person had been on this path always made me feel better. I usually look for signs anywhere I hiked, but I was alone in Chile and it got hairy for a bit.