Shows current width of the browser window in real time, to save time on responsive theme development.
Displays a tag at the top of the website that shows the current width of the browser window and the device that this width is associated with, to help with responsive design in the development stage. Helps to preview what the site will look like in mobile devices or various screen sizes. Also helps to know what size the screen to apply size specific CSS overrides in the style sheet.
Know what you are working with and save time!