Because they included this comment from Keegan Bradley, who also uses this drill a lot:
“Try and keep it [the headcover] there as long as you can during the swing. It’ll drop eventually, but not until well after impact.”It's really important to try and make the entire swing without dropping the headcover. Charley's video is one of the best demonstrations of this drill I've ever seen -- it shows her entire swing from a good angle.
This is a really good drill to teach an inside-to-inside swing, which means your hands and arms "stay in front of you" during the swing. It simultaneously helps improve your posture, your shoulder turn, your footwork and your balance. You can hit balls with it or just make practice swings at a tee in your backyard, and it doesn't take hours of practice to get better. What more can you ask of one simple drill?