The default installation of Discourse requires users to create usernames and it also offers private messaging between users. Users are not able to see the real email addresses of other users.
In traditional mailing lists, used by many free software communities, the users do see the real email addresses and they can reply personally and privately to other users when they believe it is appropriate.
With the private messaging system in Discourse, people run the risk that all their messages are seen by the Discourse administrator and anybody who hacks the Discourse installation.
While most system administrators are good people, there have been a number of cases where somebody in a leadership position in a volunteer community was subsequently found to be the subject of concern. To give an example, a senior developer left the Tor and Debian projects in 2016, the case is very widely documented so I won’t repeat it here. These things happen from time to time in any organization.
Therefore, organizations that simply don’t want to be too dependent on Discourse and their system administrators may not want the private messaging facility and may prefer to simply give users the option to communicate directly with each other by email.
People also have the inconvenience that they can’t add a contact to their address book if they don’t have a single email address for that person.