Gravestones are, ultimately, there for the living. As both a reminder and as a marking of a place of special significance to them. People are usually special to other people, and to just throw their body away when they are dead feels very respectful, even if it's just a body.
It doesn't really help people remember, in the sense that a photo would, but it does allow them to have a direct and definite place which they can connect to that person. I think that most of the time, this helps them to confront their own feelings.
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