Support links to emails (from other applications)
Applications other than Outlook sometimes need to refer to mail messages in Outlook.
For example, if I am taking notes, I might want to include a reference to an email message. Some apps will accept a dragged file and copy the contents, but many will not. In that case, an app like Mail.app will instead insert a link (a "message:" link) which, when clicked, will open Mail.app and display the message.
This method of linking to messages seems to be much more "universal" than apps that will accept a dragged file.
If Outlook supported this, one could easily reference messages from other apps, but Outlook would merely have to accept an internal reference to a message and display it, rather than generate an export of a file (that by default might not even open in Outlook if the user has multiple mail clients installed). If the internal reference is to a storage location (in, say, Exchange) then it's even usable across multiple clients.
In previous versions of iOS, internal links were supported by changing the current window focus to <#someplace>someplace: when an "anchor> defined as <a name="someplace>someplace</a> was was selected.
This is an important support mechanism for emailed newsletters.
This would be very useful in OneNote where I often want to paste a simple link to an email in my notes rather than having to paste the entire contents of my email into OneNote.