I guess the idea is that it is hard for the patient to constantly internalise the fact that the therapist is talking to him as a professional and not as a priva

I guess the idea is that it is hard for the patient to constantly internalise the fact that the therapist is talking to him as a professional and not as a private person. The patient shouldn't have to be constantly considering this distinction. Therefore, it's easier for the patient not to know the therapist-as-a-person.