What makes email more useful in general is that each email is a separate object that you can organize in any way you want, i.e. move, copy, rename, sort into folders, attach as a file to any calendar entry, todo item, etc., or indeed to any other email. You can forward them to any other recipient, you can add and remove any recipient to and from the conversation at any time. It is conceptually powerful and flexible in a similar way that files in a file system are a powerful and flexible way to organize data. And it is easy to understand.

While all of these features could in principle be realized in a chat system as well, in practice they don’t provide that flexibility and power.

Another usability feature of emails is that they have a subject line. This allows to meaningfully list emails in a compact fashion. In a desktop interface, you can easily view and visually grep 50 emails or more at once in a mail folder or list of search results (in something like Outlook or Thunderbird or Mutt). This allows working with emails more efficiently than with a chat view where you can only see a few messages at once, and only of the same thread or channel.

Yet another usability feature of emails is that each email has its own read/unread status. This, again, is facilitated by each email being its own separate data object, and by the separation between subject and body, which allows the read status to be unambiguously bound to “opening” the email, or to navigating in the list of emails alongside a preview pane. And you can mark any email as unread again. In chats, the granularity of read/unread is the whole chat, whether you’ve actually read all of it or not. You can’t easily track what you’ve read or not in an automated way as with email, other than by that coarse-grained linear time-based property of when you last visited the channel.