Extensions can use some APIs that are not available to web apps (but not all of the ones that Chrome Apps could use), and Web App + Extension is one of the Google recommended migration paths for Chrome Apps that use functionality beyond that available to pure web apps.
There is also a companion extension, but I am unsure what purpose it serves.
https://productforums.google.com/forum/#!topic/chrome/uVCvle...
https://remotedesktop.google.com/