I think that the “secret” (or at least the reason for that particular flat-region on my forehead…) is the fact that each material can have more than one texture assigned to it at the same time; each texture perhaps affecting different aspects of the material’s appearance.
You see, a “texture” is really a signal generator, and its outputs can be applied to any combination of several attributes of the material: Color, Normal, Specularity, and so-on. The influences of each texture on the various attributes can be adjusted using sliders.
So, Blender starts with the basic color and other attributes of the material, then uses the texture(s) to modify those attributes. This determines how all of the objects and faces which reference that material will “look.”
This lets you do some funky things quite easily. Like for example a really old mirror with wavy glass, or one of those gold-flaked mirror-tiles that were so popular in the Brady Bunch days. Just by mixing textures.