A headstone has one function; they are erected "In Memorial" - to perpetuate the memory of ....
That we bring the memorial to someone's attention through a photograph is simply an extension of its function.
These are our histories of those who have preceded us. They certainly do invoke images of "The way it was" ... and one cannot help but wonder at the people there and their lives.
In visiting, I am always reminded that we are finite beings; that we have little time to waste, and therefore, we should use it well.