Clicking on the bars changes your window to that size and lets you see how the layout of your site is affected by the new size. When you don't know or aren't sure what the media query points are or just don't remember, this is the best way to quickly peek into a site's layouts.
Although the Chrome Developer tools now offer their own media query inspector, it requires a page reload and resizes your page. Plus it takes a lot of clicks to get to the right place, so sometimes, simpler is better and this extension does the job quickly.
When you're navigating through different sites to take a look at font combinations, digging through the different DevTools screens to get to a font is too cumbersome. This extension makes it easy to identify fonts on sites by simply rolling your mouse over them. Clicking gives you more information about the font.
Alghough validation is something that can be handled in many places, this extension gives you one click access to the W3C Validator. It will help you quickly find errors which can potentially affect your scripts or CSS.
To test some applications properly, it's often necessary to clear things like cookies and site caches, but the process for doing so in Chrome takes a long time. This extension is a one click solution to quickly removing your site cache...set up some defaults and you're ready to go. Hit the button and the defaults will be applied immediately without confirmation (so be careful with this).