Wrap your Occam’s Razor around this.  They were markets and, more ingeniously, a tool for dividing and corralling animals.

Simon Hedger — a talented man with credentials along these lines, who works with wood (really works with it well, as opposed to dilettantes like yours truly who talk a good work-with-wood who’s never planed a plank in anger) and is based in the King Arthur Country Wales, up the road from Brynn Myrddin, where Merlin his-own-self is said to sleep away the ages — has worked out a possible (and we say quite likely) down-to-Earth purpose for your basic stone circle. 

And considering ancient tendencies toward myth-making, the circles possibly carried with them some mystical appurtenances. After all, we have the all-seeing eye on our US currency, don’t we? There is something magical about money, and it surely, like the powers in all good magic stories, can be used for good or ill.

But by and large, Stonehenge likely had a mostly commercial purpose.

Be sure to look at the Figures illustrating in great detail how this would have worked.