Answer: We don't know
Explanation: One of the biggest mysteries of Stonehenge is why it was built. Some think it was to honor the dead or for use in a midwinter festival. But the alignment of the stones to the sunset suggests a religious purpose. If your in a civilization that worships the sun, what better way in marking your relationship with the deity than have the sun shine on a particular day on your monument? It's still a riddle but we just have to appreciate the hard work by the men and women to build this prehistoric monument.