because part of that site talks about compatibility with different devices.
the phone on the left is a nexus s, which has the (now) older style of dedicated hardware buttons. the nexus prime on the right is apparently what newer devices are supposed to have, which are software buttons that are actually just part of the main lcd and the operating system reserves a section of the screen to draw them in software.
the big benefits of the newer style are that they can be hidden while playing movies or in other situations where they are not needed, they can be rotated when the device is rotated, and, maybe most importantly, the operating system now gets to control which buttons are there and in what order they're in. an annoying thing about the hundreds of different android phones prior to this was that every manufacturer seemed to put the 3 or 4 hardware buttons in a random order that made them inconsistent.
the phone on the left is a nexus s, which has the (now) older style of dedicated hardware buttons. the nexus prime on the right is apparently what newer devices are supposed to have, which are software buttons that are actually just part of the main lcd and the operating system reserves a section of the screen to draw them in software.
the big benefits of the newer style are that they can be hidden while playing movies or in other situations where they are not needed, they can be rotated when the device is rotated, and, maybe most importantly, the operating system now gets to control which buttons are there and in what order they're in. an annoying thing about the hundreds of different android phones prior to this was that every manufacturer seemed to put the 3 or 4 hardware buttons in a random order that made them inconsistent.