
A single material may make use of several textures that are all sampled and applied for different purposes. For instance, a simple material may have a Base Color texture, a Specular texture, and a normal map. In addition, there may be a map for the Emissive and Roughness stored in the alpha channels of one or more of these textures.
You can see that, while these all share the same layout, the colors used are specific to the purpose of the texture.