The early stages of the site go back to around 8000 BC although the Mesolithic posts would have been long disappeared before the bank and ditch of stage 1.
Stage 1 is generally dated to around 2900 - 2700 BC.
Stage 2 is dated to circa 2500 BC, around the end of the late Neolithic. During this stage the bluestones were moved from by the ditch to around the inner Sarsens.
Stage 3 is dated to relatively soon after, circa 2400 BC as the copper age ushers in the age of metal. This is also when the Avenue appears to have been added.
Stage 4 is circa 2100BC and is in the Early Bronze Age, (EBA) the time of the round barrows appearing in numbers.
While the building of early monuments such as the large posts in the Mesolithic were built by indigenous hunter gatherers, who used the area before Stage 1 of Stonehenge, which follows the introduction of farming. Small scale immigration or influence from trading may have introduced new ideas that were adopted by the local population. The introduction of farming also introduced the beaker culture.
The evidence appears to be give at least two answers, the main answer appears to be as a place of veneration for the dead, possibly with Woodhenge as a place for the living. Certainly Stonehenge has a place in the wider landscape hence the Avenues leading the river Avon from Stonehenge and Durrington.
The alignment on the winter solstice, while marking the shortest day, may have had implications for farming and the society involved in that, but may also be the date that the dead were venerated during a time of feasting and coming to together of groups.
I have never liked the glacial deposition idea as it just doesn't fit with the evidence. I think that more evidence is required but I do like the idea of stones being used as ball bearing in sleds. I think the raised platform idea for the lintels is quite possible but does seem wasteful for resources and unless the Timbers are packed with earth or are shaped and joined, they could be slightly dangerous. But it does seem most likely as there is little evidence of ramps or revetments, or holes for A frames.