Why do we put a stone on a grave when we visit it?

A stone on a grave makes the point that you have visited the grave and furthermore builds up a matzeva. Once, the matzeva was not the concrete slab that it is today, but made up of a pile of stones constantly added onto by people who visited the grave and placed stones on the grave site. Without the additional stones, gradually the wind and weather etc. would cause the grave site not to be noticed.