If you build browser-based applications that open websites in new tabs and programatically interact with them then I imagine it'd be handy. I'm making a test builder to generate user scripts and e2e tests more easily, so having the app open in one tab talking to a site you're working with in another would be a possible use case. I've actually written my app as Chrome devtools extension though, so it already has access to everything through a background script and an injected content script. If I ever move it to being web-based I'd use something like this to do communication between tabs.

Looking at Wingify's website it seems they're doing something similar with A/B testing - they (presumably) have an app that sits in one tab and a two of copies of the target site in two more tabs, so changes made in their app are reflected in the two versions of the target site. That's a total guess though.