If you assign the textures to the wall components, you have a better way to make sure that you get the correct texture on the wall. The other way to assign textures is to assign textures to the overall part of the wall. The downside of using overall, is that you have to know which is the left and which is the right side of the wall. The left and right sides of the wall can change depending on the direction you draw the wall. Assigning textures by the components removes this problem, regardless of the way you draw the wall the components always have the correct texture.