When you create a layer, a separate material node is automatically generated and linked to the layer. Every layer must be associated with a material, but many layers can share a single material. Because the material node is separate from the layer and surface, you can easily propagate material properties to several layers from a single source, instead of duplicating effort by having to assign the same material attributes to several surfaces. You can quickly connect materials to layers from the Reaction Group Schematic and see which layers share materials.