It's for building Web applications that can be modified independently of the host service by the users. Think of applications that are plugin heavy - Emacs, Vim

It's for building Web applications that can be modified independently of the host service by the users. Think of applications that are plugin heavy - Emacs, Vim, Sublime, Photoshop, 3dsmax, MS Excel (in its way). Similar things could be done with, for instance, user-pluggable forums. The goal is to use Worker VM containers and permissioning so that plugins are safe to share between users.