This feature may seem like a simple nicety, but for beginners, this is a bigger deal than it may look like at first glance.
You see, while for most developers, large monitors with plenty of screen real estate are the norm, for beginners, that doesn’t hold true. Most beginners, myself included, are using some variation of either the Macbook, Macbook Air, or Macbook Pro, and while the 15” Retina Macbook Pro offers a reasonable scaled resolution of 1920x1200, most of the other panels in the lineup have pretty limited screen real-estate.
Regardless of screen-estate though, with any of these Macbooks, when you place Xcode and a book side by side, things can get cramped, very quickly. As a result, embedding learning content within a playground goes a long way in improving the learning experience.