Having a basic artistic knowledge of anatomy helps you see what to paint.
Without some knowledge of anatomy the back of a person just looks like a bunch of lumps and bumps. Of course for this figure almost everyone can pick out the rib cage so that should be easy. But when you get into the lesser known parts like the scapula, trapezius, latissimus dorsi, external obliques and more then some knowledge is needed to help guide where the pencil or brush goes. I find it interesting, and hard to describe how, that having an understanding of what your looking at really helps you to draw and paint it better.
If you know the forms under the skin its easier to paint the volume of those forms. Also if you know where the bones are closer to the surface of the skin this can change what colors you choose for that area. It’s all helpful, but you don’t have to know everything by heart or with a surgeons precision, just a few lessons go a long way.