Advantages

■ It allows unparalleled unobtrusive data collection. Because the moderator is not in the room, this eliminates almost entirely any  possible biasing effects due to inadvertent nonverbal communication or mannerisms.
Being in a (nearly) soundproof room, the testing team and observers can easily communicate among themselves about the proceedings and discuss possible product improvements.
This environment can accommodate many observers (typically 4 to 12) viewing the test at one time.