Remembered

So Rachel died, and was buried in the highway that leadeth to Ephrata, that is Bethlehem. And Jacob erected a pillar over her sepulchre: this is the pillar of Rachel's monument, to this day. — Genesis xxxv:19-20.

That was the first biblical account of a grave marker/monument. Now, it has long been Jewish custom to leave a rock/pebble on a visited grave. In a very small part, it harkens back to that first stone. It also shows the current visitor has not been the only visitor. The physical act of bringing, holding, and placing a stone on a grave causes a memory to be installed in the visitor of the visit. In a vein of Jewish thought this is a ritual; in Catholic thought such an act would be likened to a sacramental, such as dipping fingers in a font of holy water to make a sign of a cross. In another way, it is as if each successive visitor helps to create a cairn to keep the grave.

Some maintain this deposit of stone comforts the deceased. In most graveyards, after a time such stones are removed. I do not know at one point, perhaps someone may answer.