Their names are pretty self-explanatory They allow you to move the origin and the extent of the viewport and window around, and enable you to set the extents as a logical scale rather than a pixel one. You might use them to display graphs or graphics, where changing the origin would make the calculations easier (such as using a Cartesian co-ordinate system).
Petzold goes into quite a bit of detail about them, what exactly is confusing you?
