Currently your choices are emacs/vim over SSH or sync from your local machine (where the code intelligence / debugging integration probably doesn't work as desired).
A web IDE allows an arbitrarily powerful cloud instance to do the heavy lifting of build/test/code intelligence while the developer still gets a full-featured GUI IDE. Especially since VS Code and Atom are Electron apps, it's a natural step to use them to edit remote files. Better than VNC, anyway.