Coloring on seperate layers

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Imagen de spadekevin
 I understand how layers work from my experience with Photoshop and After Effects but what I don't understand is why one would choose to put their color and shadows on a separate layer from the outline of their drawings. Is this for better control or effect? I know that Sandra Fierlinger uses layers of color to simulate water color transparencies, so that would make sense. But in a situation where one is just filling in straight color, what is the purpose of putting in on a separate layer? 
Please help.
I did not find the right solution from the internet.