So far, the blog list example has assumed a single layout for all devices and orientations. Of course, we know that some phones are much larger than others, and tablets are larger than phones, and landscape vs portait orientations provide for very different amounts of screen real estate to work with. Having a single layout accounts for none of this, and inevitably forces the design to the lowest common denominator (a small phone in portrait orientation).
Providing different layouts means having reusable UI components becomes very important. In Android, these are called fragments. The Blog List app follows the typical example for which fragments are useful: a list of items that provide additional detail about a selected item.