There are several reasons for photographing headstones. Outside the United States (U. S.), a number of countries or locations re-use graves if land is limited for new burials. In those cases, the headstones are often removed. Germany, Sweden, among others, re-use graves. In the U. S., it’s pretty rare to re-use occupied graves. Stacking is becoming more common in some cemeteries – https://www.atlasobscura.com/articles/london-double-sided-graves, both in the U. S. and in other countries. I read an article years ago where some Veterans Administration (V. A.) military cemeteries had started stacking because they were running out of space.
Another reason to photograph headstones is time-related. Over time, most headstones become unreadable due to various reasons. I was in the second oldest cemetery in Salem, Massachusetts in 1999. Many of the stones were unreadable. I was on a short break so I don’t know if the cemetery had records for those graves or if the records were still readable if the cemetery had records.
A third reason is headstone loss. My great-grandmother (mother’s father’s mother) is buried in the same lot as my grandfather, her oldest son. She died in 1926. At that time, she had a headstone. However, she’s buried in Hurricane Alley – . Last time I asked somebody to photograph her headstone, it was missing. I believe it’s been missing since 1969 or earlier. Was it lost during a hurricane, is it simply buried in place, or was it stolen? I tend to think hurricane or buried in place are more likely than stolen.