This is important for a few reasons; say you have a cook book, and you want people to be able to jump to the section on Cookies or Chicken. (goodereader.com)
This is great for getting an overview or for jumping to a section for a review. (hawkeyemedia.net)
Why it's useful: The app itself tracks your progress and is built so you can jump to sections like learning the basics for asking directions, ordering food, and general travel. (adventureinyou.com)