The ability to add User-Defined Unified Folders.
Example 1: People are sent receipts and registrations from many places. They set up a "Receipts and Registrations" folder. These need to be able to be on line where they are backed up and available from multiple their devices, something local folders cannot provide.
Example 2: Adding to Example 1: Small business has visibility of multiple email accounts for employees. They check "Receipts and Registrations" for the week to post to purchase orders or to reconcile credit card accounts, plus maintain proper documentation for an accounting audit. They can search across multiple accounts for the paperwork.
Example 3: Users may have any number of folders that they add to each account that they want to see the contents consolidated / unified for. People are getting more and more email addresses these days. I've notice people have searched a lot for this feature.