There is something very satisfying about the elegant simplicity of a standing stone. It is a clear statement that people put it there - while you find many stones lying around in the landscape you don't find them standing proudly on one end! Because of this stones have been used as markers throughout history as boundary markers, milestones and the like. In historical times, of course, we have the benefit of writing - on the stones themselves or elsewhere, to put these stones into context.
But when it comes to the question why did Stone-Age man build many megalithic structures in the British Isles no simple answer emerges. That the reason was good one in the Neolithic people's eyes is in no doubt - modern experiments finding the effort required to construct a stone circle show that a large proportion of the surrounding population would need to be involved to raise enough manpower, and to feed them and make up for the loss of their farming effort.
Too often the presence of burials is used to make the assumption that Neolithic craft was mainly funerary. This is perhaps a reasonable assumption for the many barrows that very often contain human remains. On other sites, however, the presence of bones does not always mean this is a tomb - there are plenty of bones in many churches, and yet churches are not generally tombs.
Because of the large effort needed to raise them, it is reasonable to imagine that some collective ceremony took place. Other standing stones are aligned towards events in the complex lunar cycle, or towards the midwinter or midsummer sun. Too much has been lost about the human context of many sites to be able to say with any accuracy why they were raised, and it is particularly the simpler monuments, the single standing stones, that defy explanation the most.