as there is no written record as to the original purpose of Stonehenge all answers will be supposition.
It has been suggested as an interplanetary landing spot for ancient aliens, most unlikely to being an unfinished tomb for an ancient warlord or king; possible.
The most likely is that it was possibly a solar calendar, the alignment of certain stones make predictions of the longest and shortest days of the year clearly predictable.
A major debate now rages as to whether the solar solstice at mid summer was the main celebration, or the midwinter solstice.
Personally I would go with Stonehenge being more important as a marker for the midwinter solstice. For an agricultural society knowing when to prepare to plant crops would have been more important against knowing when the longest day had passed, there is little reason to my mind knowing the longest day as this occurs at one of the periods of activity for farmers and any farmer would have been able to assess the status of his crop by looking at the crop not by looking at Stonehenge.