An even better solution to coralling, although maybe not herding, is a henge. Lots of animals don't like crossing a ditch, especially if it's a moat. Hmmm, right?
Right. It all comes down, doesn’t it, to how much energy you have to expend to achieve whatever it is you want to achieve. If digging a (circular) ditch is easier than dragging stones around, hauling ‘em up and then filling the spaces between with something then that’s what you’d go for.
At the risk of sounding sacrilegious I’m just vary wary of assigning a sacred significance to every stone circle when some/many/most? were purely for utilitarian purposes.