Disclosure: I'm a developer of this tool & work for BBC News.

We use this for catching things that we'd never spot, a good example of how it has helped us is after a giant Sass refactor, we ran this tool against the old CSS site and the new CSS site, this showed us where things were off by a pixel or two (as well as big formatting changes).

We've found this tool rather valuable, we're not saying everyone will, or that others should use it. We open sourced it because it might help someone.

To catch unintended effects of CSS or HTML changes across a site without having to manually check every page and try manually spot differences.

I think the value is in scale. We're currently working on a very large product site (not eCommerce) that has thousands of products and thousands of related content entities. QAing that is a nightmare, but once you get through the base pass, imagine rolling out changes. Being able to run this over thousands of pages, seeing the potential changes over time, will be hugely useful.

This is extremely useful if you are trying to build a responsive site. Imagine you have to test 10 different screen resolutions... even using an extension the process would probably take up to a minute where as this is just running a command. Now imagine the process repeated X times...

It's an automated testing tool so you can compare the site to a bunch of screenshots you know are correct and see if anything is wrong.

Pixel perfect on all browsers?