Why was Stonehenge put on the world heritage site list?

Stonehenge is important because no one knows why it was built or who built it. Also the stones used to build it were an enormous size for the technology around that time. How they lifted those stones or transported them from their distant location is still a mystery; much like the pyramids of Egypt. People do not know what its purpose was. Some believe it is a calendar or a burial site or a place for religious sacrifice.