Main point to me would be that it would work automatically on every platform and without needing setup/installation/updating, so I could switch between a Chromebook, a Mac, PC with Windows or Linux without even having to think about whether the IDE has been ported to that platform. If there would be a practical IDE solution running in the browser (or Chromium as offline package) for C++ which replaces VStudio/Xcode/QtCreator I would switch in an instant.
But what is the purpose of creating yet another Web IDE (for Go) if there are already many established open source solutions like Eclipse Orion? They could just add a Go code completion and a build service to it.