Grayscale

Whats are the advantages of using it?

A couple reasons are it's simpler to shade when you aren't worrying about the colors at the same time. The grayscale skin is like just a "shading layer" then you just add your extra details and coloring later in new layers. Once you have a grayscale skin it's easy to make different variations of the same skin to choose from as well because when you get to the coloring and detailing phase you aren't thinking of shading, just making colors.
That's just a couple. I'm sure others can add plenty of other pros about it.