What is the purpose of a shoulder check?

• To see areas not visible in your mirrors.
  Explanation:

  "A shoulder check helps you spot blind spots that mirrors cannot cover. To do it correctly, quickly glance over your shoulder through the rear side window in the direction you plan to move. This ensures safer lane changes and turns."
• To view the entire area behind the car.
• To see what is happening in front.
• It serves no purpose.