I agree with Bohemian Princess. I think some therapists use this approach because they want to remain completely objective. If a therapist shows emotion while the patient is expressing themselves, this could affect a vulnerable patient who's impressionable. It's like when professional poker players are at a table - hence the term 'poker face'. I think it depends on the patient too - some patients don't mind just talking and not getting much feed back and others need that interactive response.