There seems to be more subtlety here than is explained in the book. I didn’t really understand what the point of these buttons since the app still ran with a landscape display even if only Landscape Left and Landscape Right are enabled and the device is in portrait orientation.
The subtle difference, seems to be if you have the device in portrait and only portrait button is enabled, then when the app launches the status bar remains at the top of the screen. When the HeavyViewController.xib loads, then the status bar changes to the side and the view is shown on its side. That change in the position of the status bar looks ugly.
However if you have Landscape Left and Landscape Right enabled, and the device is in portrait, then when the app launches, the status bar immediately gets shown on the left of the screen and the xib loads as before.