>>2415914>>portaging and looking for the trail headCairns are different. Cairns are built to indicate a route across a rocky area, and are really important, both above treeline and, as you say, to indicate a portage route.
Most of the times, "stacked rocks" are a kind of aesthetic vandalism. My theory is that true outdoors-enjoyers would never stack rocks, but they might bring their girlfriend or children, who might get bored with just the woods, and they are the ones who stack rocks.