Aliens did it - why? Was it like some sort of summer home? Why not stay in their (presumably) safer spaceships? It's not sterile, so I can't see it being used as a surgery/laboratory. We The locals did it - how? It'd take precise technology, a lot of time, effort and skill. And to what purpose was the place put? We've dozens of ideas and legends about Stonehenge, but what about PP?
Hoaxers did it - why? Ohh this is the easy one, to make it look like aliens did it.
All very good questions. Only one thing stands out to me as rational. That is that they must have had a definite purpose in mind; functional in some way to produce a profitable return.
In the same way as we build hydro dams for a return for example.