2. Posture training: By pinching your shoulder blades back and down, you get an added bonus of firing those scapular muscles isometrically. Any work on those scaps is great for posture.
3. Balance! If you perform these Cossacks repeatedly on the same leg (instead of alternating legs, marching style), you will allow those small muscles in your foot and lower leg to work hard to maintain balance on the grounded leg. You will find that your hamstrings and adductors will fire as well to keep that leg from wobbling.