Of course he wants a unique key for you in his patient data base - too many John Smiths. And, this key has the additional benefit in being useful in other data bases here and there. He may want to validate some of the personal information you have provided. He may check your medical records with other health providers. He can check your credit before ordering the manufacture of expensive bridgework. He can check to see if you've had a malpractice suit against another doctor.

But, you knew that.