Why is everyone* on a stone indexed (also: Why are relatives indexed)?
In short, for genealogical purposes. Some headstones provide family relationships and this information can link previously unknown relations. For example, a headstone may state "Sarah Russell daughter of John Russell". If there were no other record of Sarah, her headstone would be the only one to link her as a daughter of John. By indexing John and linking him to this headstone, it lets those seeking him also find Sarah.
This type of indexing may sometimes mean one person will end up in our index multiple times. That's okay as long as each time they are in the index they are linked to a different headstone. One photo may show their headstone, one may show them as a spouse, five may show them as a parent. But they all help researchers find every stone a particular person is named on.
*The exception is living persons.