Viewed purely as an image manipulation tool, I have real trouble seeing a good reason why this couldn’t be done locally using Cairo. Viewed as an Imgix parameter previewer, I don’t see why you wouldn’t simply use Imgix directly via a web browser.
Yes, you can use imgix directly via the web browser, but for building complex URLs this is way faster (especially if you're an experienced emacs user). Think encoding and moving between parameters of a super long URL.
I don't see why I'd want to do it in a web browser when I can simply do it directly in Emacs :)
I don't know much about imgix, looks like a service to add to or possibly enhance images? Demo looks neat though! I do use emacs as an asset browser for quick looks at images to verify content in web apps, but I'm not sure I'd use something like this personally.