The same question can be asked in this case: if there's no money in it, why did Microsoft pay for the creation of a new Outlook that's apparently worse than the old Outlook?
I think it might be less about making money and more about burning less. From what I understand, the new Outlook is a wrapper around the code that already powers Outlook.com. Since they're already maintaining Outlook.com, they might as well integrate the functionality of the Office Outlook into it and only maintain one of the two moving forward.
Actually the answer is quite simple, their servers are the ones retrieving emails so they have access to the email contents (perhaps for training data?)
Prior to this new version, Outlook did not pull emails through Microsoft servers but pulled them directly from the mail server configured. Now, when you retrieve email from Outlook New, Microsoft servers are "proxying" the emails through their systems.