We can’t know for sure because the people who built it didn’t leave any written records. But Stonehenge isn’t one thing. It is a sacred site that has been continually added to and rearranged for over 5,000 years. The earliest construction in 2900 BC was a circular bank and ditch, with an inner ring of large timber posts.
Three hundred years later, a ring of standing stones was added, using stones dragged from Wales, 240km away. Then a hundred years later, the ‘trilithons’ (standing stones with lintels) were added. Even after that, there is evidence that the arrangement of the stones changed several more times.
Like the O2 arena, or Trafalgar Square, Stonehenge was probably used for several purposes at the same time. It was a festival venue, temple, burial ground and astronomical calendar.