Some reasons brought down in various sources:
1. As a sign that someone has visited.
2. If one inadvertently read the inscriptions on the matzeivah, putting a stone on it mitigates the possible loss of Torah learning such reading might cause.
3. As an invitation for the nitzotzos of the niftar to descend during the visit.
4. As a kapporah for the niftar in case he was chayov sekilah.
Can someone please ask him what the makor for putting a rock on the matzevah is?
Some reasons brought down in various sources:
1. As a sign that someone has visited.
2. If one inadvertently read the inscriptions on the matzeivah, putting a stone on it mitigates the possible loss of Torah learning such reading might cause.
3. As an invitation for the nitzotzos of the niftar to descend during the visit.
4. As a kapporah for the niftar in case he was chayov sekilah.
Heavy heart