When you set up an email account, you rarely have to enter the server details. This wasn't the case a few years ago, when you had to choose between POP3 and IMAP protocols and enter the connection details manually. The first meant your messages would be deleted from the server and stored locally, while the latter implied emails would remain on the server. In today's world, it's almost unimaginable not to use a server-oriented setup, but Microsoft begs to differ, as it just added support for POP3 in Outlook mobile.
Most current email services rely on Exchange or IMAP to sync your messages, mainly because we check our mailboxes from different devices, meaning items should remain on the server to be accessed consistently. Unlike IMAP, the way POP3 works often implies messages are downloaded onto the device and removed from the server, which won't work if you want to check your emails on your both your computer and phone, let alone on your tablet.
Nevertheless, Outlook mobile just gained compatibility for POP3, as some services may not work with IMAP. This "new" addition doesn't necessarily bring value for most users, and I'd highly recommend sticking to IMAP or Exchange in your configuration, but it's always convenient to see an app become more widely compatible with existing standards.