What is this pipe coming out of washer dryer box?

[Bryce Y.]

See picture below. The tenant told me recently that there has been a slight constant drip coming from the PVC pipe in the middle. It is draining directly into the drain line, so there's no concern of water damage. I am just confused as to where this pipe leads to and its purpose. On the opposite side of the wall is the bathroom, but the sink and shower do not share this wall. The house is 1 story.