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.