Some somatic practitioners have suggested that turning that back has a profound physical relationship to feeling the direct support of two people instead of always trying to protect your back.
Also, turning the back doesn't just heighten listening, it prevents you as the listener from influencing the dialogue with expressions of agreement or body language that suggests disagreement. Also, it enables the consultants to really talk to each other instead of talking at you, the client.
Other than just forcing the client to listen, is there any other purpose to having them turn their backs to the consultants?