When I encounter an hr:manager property, I know whether I know what it means.

Everyone can create such properties without congesting a central registrar.

You can create a my_hr:employeeName property based on hr:name.

You can verify that some data uses my_hr:healthPlanCode consistently with the rules for hr:healthPlanId.

Schema languages, for instance W3C's XML Schema, provide analogous constraints on syntax -
 but they are harder to use on composed documents.