Which begs the question, why does the Bishop have to contact the person to make certain? The fact that he/she took the time to write the letter or make the request should be accepted at face value and honored.
To be honest I am not sure. Perhaps for the same reason federal agencies verify. To make sure the request came from them.
99% sure that is the reason. Anyone can write a letter, just verification that the letter did indeed come from that person.