Painting realistic texture can be a challenge. It's especially challenging when you're drawing objects or characters from your imagination. This exercise is designed to help you strengthen your mental texture library. The most important part of this exercise is the mental extrapolation. Often times painting from reference materials is a 1:1 copy, which doesn't force you to fully examine your subject. This exercise requires you to do more than copy, and strengthens your mental texture library in the process.
In school you've probably grown to loathe the term 'study'. When it comes to math and history, this term is synonymous with painstaking memorization. In the case of art this term can take on a slightly different meaning. On ctrl+Paint I often refer to 'studies', and this video helps to explain my meaning. Though your version might take on a different form than mine, 'studies' are the way you improve at art.