Let's say you're creating an elephant or a dragon. These creatures will have all kinds of wrinkles, creases and bumps in their skin. I want those creases and to sometimes have a different color than the rest of the mesh, like being darker or lighter.
So when I'm sculpting or making materials, how do I tell blender to make part of a mesh a different color based on its distance from...er...hmm...distance from what? I don't know...a low poly base mesh? Hmm...well you see the dilemma right? If I want a creature's skin to be one color, but then I want their scales to be another color, how do I control that?
If you are going to Animate a Sculpt, you'll have to re-Topologize and then you can Assign different Materials to different parts of the Mesh.
You can also Paint in Sculpt Mode.
If you have used Multires, or have a Low Poly version of your Model, you can Bake a Displacement Map, which you can use to 'separate' parts based on their 'elevation' from the Low Poly version.