Although this house was built without plumbing, the Hermanns and Grimas might have kept a room like this in order to facilitate bathing.  Tubs could be stored and used in the bathing room rather than carrying them and water through the house.  There are two entrances into the bathing room, so slaves could bring the water in through the gallery or the alley, carrying it over stone or wood floors rather than over expensive carpets.  A drain funneled water outside the door, down the alley, and into the street, allowed slaves to empty the bathtubs easily, without having to move them over long distances.