Even though your bathroom walls may look square, every wall isn't perfectly level. Homes shift, and the walls can bow out in sections which makes tiling more challenging. In contrast to materials that follow the contours of the wall, metal lathe and cement can be shaped by hand. Any bows or valleys in the wall can be leveled by adding or subtracting cement.
Metal Lathe and Mud was used in the early years in home construction. If you have metal lathe and mud in your home, there will be a lot of dust during removal.