This is the simplest way to present what I mean when I say the design is responsive. This refers to the size of the screen your app is on. In the example, you can see how the app not only adjusts its size, but selectively chooses design elements to display depending on how much screen real estate is available.
Beyond being simpler to create, it makes it easier to use the services on different platforms. In the end, everything will work the same way and be in the same place. Right now, for instance, Gmail is totally different on mobile than it is on the web. With the Material Design makeover, it will be essentially the same thing in all formats.