Thats the thing. How do we know what these mean. It could have been some dude just saying you know what throw them rocks over there anyway you want and be done with it.
It could have been many things, but not that. The stones were worked, transported and placed. Stonehenge would have required a great deal of time and effort, so it wasn't anything casual.
If I had to put money on it, I'd go for a holy site. Stonehenge is aligned with the sun at dawn on a solstice - maybe it represents the union of the earth mother and sun father in a duotheistic religion.
Or maybe it was tribal competition. There are many henges in Britain. Maybe one tribe built Stonehenge to be more impressive than the others as a sign of greater wealth and power.