The Importance of Mirrors in Interviews

I used to work in market research my first job. It was with women between the age of 18 and 39 who had acid reflux. And you watched them through one way mirror. There were people from, I'm not going to say which pharmaceutical giant were watching them through the window,. The lights are always dark. You don't know whether there's someone behind there or not. If you turned on the lights in that room, you'd see them. What's the point? Because they wouldn't have otherwise.