There's plenty of rock cairn trigs around but building one on top of a giant rock is something a bit out of the ordinary, I'd give points to whoever built this one.
It’s more common than you might think. That should be observable from a wide range of locations around it.
I’d use it as an RO. I’d be happy to throw a set of legs and GPS antenna for a base over or near it as well.
My guess as to who placed it would be the state survey department.