These pieces of stone are for the interior walls, every one numbered and catalogued so they know exactly where they go. The Parthenon is sort of like the world's greatest jigsaw puzzle. Notice the smooth columns. The philospohy a few years ago was to make it obvious what was ancient and what was new stone. But then they decided that this was a little too obvious. In fact it was kind of ugly. So supposedly they will strip these down and create lines to give the impression of the fluted columns.