Some devices and programs are not compatible with 2-factor authentication (i.e., those that use IMAP). If I set up 2-factor authentication with OTP, that means I still need to allow the regular password to be used for places other than the web login. To mitigate this, mailbox.org could offer "app passwords" that are randomly generated for a device. Thus, the regular password could be completely disabled when 2FA is enabled. OTP would be used for the web interface and app passwords used for other programs/devices.
See examples here: