The storage event is still useful for sessionStorage within the same tab for some uses. For example:

If a webpage has more than one iframe in the same tab and has the same sessionStorage, if one iframe may change the value of the sessionStorage, other iframes in the same tab can be notified to do something.

In other words, a storage event for sessionStorage is there when multiples of the same document would share the same sessionStorage for one tab and not for cross-tab communication.