The overhead squat is a compound exercises in that it works several muscle groups at once. In the training do for myself and my clients I focus almost exclusively on compound exercises. We use it regularly in CrossFit. The benefit of compound exercises is that they are similar to how we move in real life, they allow you to work several muscle groups at once, and are more applicable to athletic performance than isolation exercises.
The overhead squat works multiple muscle groups at once, and will dramatically improve your form and strength in standard back squats, as it trains you how to maintain a vertical torso and forces you to keep your weight on your heels. It also provides a great stretch for the shoulders and upper back while working wonders for the abs and lower back, which are engaged from start to finish controlling and stabilizing the weight far above you.
How to perform an overhead squat
Benefits of the overhead squat