Does this add anything above and beyond the already included developer tools, which I already access with ctrl+shift+I and ctrl+shift+J (on the Linux dev channel)?
Yes, it has options similar to the Web Developer extension for Firefox, such as disabling specific stylesheets, resizing the window or outlining block level elements (which I've always found to be very useful).
It's not as nice/refined as the Web Developer extension of course, for example that 'outline block level elements' readjusts the whole page which it does not do in firefox (which is why I found it useful in firefox, it didn't actually add a 1px border).
OT, but that is the worst "lightbox" image viewer I've seen. You have to actually click the little 'x' to close the dialog (as opposed to most implementations that let you click anywhere-else to close it, or hit escape).