Here is a good /r/askhistorians thread for essentially the same question.

/u/antialcibiades recommended this episode of Nova on the topic. I guess I can put his TL;DR here, but there is a lot of info that thread.

The theory that Nova runs with the most is that Stonehenge was actually one of two complementary neolithic monuments. Stonehenge, made of stone, was likely associated with death; nearby Durrington Walls, made of wood but otherwise very similar to Stonehenge, was associated with life. The two monuments were linked by the River Avon and the ceremonies at the site could have included a symbolic passage from life to death. As well, the monuments are aligned with the summer and winter solstices. Thus, it is believed that people came from all over on the longest and shortest days of the year and held ceremonies of death and birth.