The Responsive Design View makes it easy to see how your designs will look on different sized screens. Responsive designs adapt to different screen sizes to provide a presentation that is suitable for different kinds of devices, such as mobile phones or tablets. The screenshot below is an example of the mozilla.org website viewed with a 320 by 480 content area.
The Responsive Design View is convenient because you can quickly change the size of the content area without changing the size of your browser window. Changing the browser window's size can be annoying because all of your tabs also change size, and you have less space in the window for developer tools.