Depends where you are. There are many reasons why this might be a valid thing to do.
TRE for the authority not rated in type.
Registry for the machine does not match the licence of the examiner.
Owner of the machine doesn't want anyone else but their own pilot as safety pilot.
The machine is military-owned, but a civil type.
Insurance.
And on and on.
I've heard of, been in, or seen each of these scenarios.