To clarify:  Fundamentally, this hook lets you change what `\OC::$session` is.  `\OC::$session` is still used for all session storage from OC's perspective.  The implementation of a custom class to replace the default instance of `\OC\Session\Internal` for `\OC:$session` with something that extends that base class is what allows apps to implement custom storage for session data (via custom PHP session handler, or simply directly).