Allegedly, It's so they can use push notifications efficiently from a central place for new mail notifications. Mobile IMAP clients cannot reliably keep a TCP connection open, due to on-device power-management and network management purposes. IMAP natively supports push-based notifications, but mobile networks make that impossible to work reliably.

Which is odd. I also have Outlook for iOS, and the accounts I add in Outlook for Windows don't automatically appear in my Outlook for iOS, even though I have the same Microsoft account logged in in both. So I don't understand how they plan to make push notifications work better for me on my iPhone by relaying my IMAP e-mail account.