True, But they also do much more things than Emacs does.
It allows you to embed GTK widgets INSIDE an emacs buffer, effectively allowing one to create actual GUIs inside of emacs.
This also allows you to embed a full webkit-based browser (with javascript support!) right into emacs, and use it like a regular browser.
Just clone the emacs repo, build it with gtk3 and xwidget support, and run 'M-x xwidget-webkit-browse-url' to get a full-fledged browser with proper rendering right inside emacs.
https://www.emacswiki.org/pics/static/EmacsXembedScreenshot....
https://en.wikipedia.org/wiki/Eww_%28web_browser%29
I don't believe there is any Javascript support, though, so you'll be out of luck for many modern pages...