I don't always do this, even though I should. A grayscale image of a piece can help define things that the ink lines would make too obvious. Also, for something like the background, I'd have to do a lot of texturing if I left it as an ink drawing. Here, I can just fill it with a gray and see if that pushes the background back and allows the important element to come forward. It doesn't take very long in Photoshop, so it's worth doing.