For example, the Elm community has both. The Discourse forum is technical and business-only yet there's a clean record of these discussions. The Slack chat is where I hang out, get to know people, and participate in more relaxed chit chat about Elm, webdev, and building applications.
Elm used to just have a mailing list but it was obsoleted and shut down with the creation of the Slack group and Discourse forum which were far more popular.
Any community that only has a mailing list could benefit from experimenting with a proper forum. I've seen this experiment broaden a community time and time again as you move away from only selecting for the type of person who likes mailing lists. And notice that HN isn't a mailing list either.
For example, I would imagine that the sort of people interested in high-quality ebooks extend beyond mailing list loving super-techies. Even a subreddit would be a nice option.
Some people might say it's useful and others might say it encourages people to comment first and possibly focus on polishing the comment second.