The glutes are a major muscle group. They power the lower extremity and connect the lower body to the core. Because of this, they represent the most powerful and important muscle group in the body.
Sure, there are other important muscle groups, but nothing unleashes as much power and protection as the glutes. However, if they are tight and inactive, the system becomes extremely vulnerable.
This stretch helps lengthen the glutes. Doing so provides for more efficient usage and protection to the knees and low back.