This is a very difficult question to answer, because stonehenge was build so long ago. Through radiocarbon dating, scholar believe that stonehenge was build in the neolithic time period around 2500 BC. By this fact alone, it is difficult to say anything about the building of Stonehenge or even what is was used for. Time has a way of making things obscure.
Some people have posited supernatural methods or anachronistic means of construction, which should be discarded. The best explanation is that Stoneheng was built through the use of dirt ramps that were placed on the side of the pillars. By doing this, they would be able to push the other stone on the top. Human exertion and effort goes a long way in building things like Stonehenge.
It is likely that we will never know for sure how the people who created Stonehenge managed to do so without any modern technology. Scholars guess about this sort of thing all the time, but there is no way to definitively prove how it was done.
The biggest mystery surrounds the way in which the stones were transported to the building site. Scholars have speculated about various ways of making this possible. They have speculated that logs were placed on the ground in the path of the stones and used as rollers to allow the stones to move more easily. They have speculated that the people set up wooden tracks and pulled the stones on sleds over the tracks. These tracks might have been lubricated with grease.
Once the stones were at the site, one assumption is that ramps of earth were built so that the lintels could be dragged up to be put in place.
All of these are informed guesses. There is no way to actually know for sure.
According to Celtic legend, Stonehenge was erected by the wizard Merlin. He waved his wand and stones from far away hovered into place.
It was built precisely with the observation of the lunar and solar cycles integrated into the arrangement, as well as the tracking of Precession: a phenomena defined by the horizontal shifting of the rising sun by 1 degree every 72 years.
stone henge was built actually as a mounment for life & death, but stone henge was built to represent immortality another stone henge was said, by legend to have built of wood which represented death. I think nobody knows how stone henge was built some say, they where primitive people who knew nothing actually to move stones you would have to have some help possibly other worldly help. I think they weren't stupid it's probably knowledge that we have lost over the centuries. Monuments where built in stone for a reason mainly to send us knowledge or possibly a warning their are videos on the history channel.com that I reccommend you to watch called "Ancient Aliens" it tells the many possibilties on how the monuments of our world could have been built. Their is no answer to how they where built many where built for knowledge others where built to warn us. this is basically why I like history because nobody and I mean nobody at least in modern times that knows how these structures where built we only have theories "some crazy theories". I love this question!