The Preselis are the probable location of the quarry from where the bluestone was mined used in the second phase of the construction of an unusual stone circle in Wiltshire called Stonehenge. Seeing as how this is the only location in Britain the rock is found (a particular form of Blue Dolerite) one has to ask why did this rock had to be used, and if it was important then why that location (it is 135 miles away after all).
As an aside the bluestone was used for only part of stonehenge. The original site first existed as the earth bank and ditch enclosing a central area containing four stones at the solstisial points. Several hundred years later bluestones were brought and arranged in a circle within the henge. These stones are all about 3 metres tall (some part of which is below the ground) and have no lintels connecting the tops of the stones. That bluestone circle was dismantled and then partially re-erected as a different circle on the same site during the last phase of construction (over a thousand years after the site was first developed). This final phase in the building of stonehenge included the erection of the famous, characteristic and unique sarsen stone pillars topped with lintels.
For a while a prevalent theory was that the bluestones used in stonehenge were glacial erratics. That is bits of rock carried from some distance away by glaciers and dumped in a location where they would not normally be expected to occur. This theory has pretty much lost credible support as there appear to be no other similar erratics in the area - only those stones used in the circle itself.
A mate of mine proposed the most plausible explanation I have heard. He reckons the people whose circle it was, originally lived out in Pembrokeshire near the Preselis. They subsequently moved into the interior of the mainland, and took their most important belongings with them, including the circle. The move could have been prompted by climate or food source reasons. He may not be the first to propose this explanation but it was an original thought on his part.
It was either that or maybe it was only bluestones which were magically resonant in the right astral fields to adequately channel the orgone energy for the bronze age man's love temple.