Subscribe to code changes
It is often the case in a big team that you work on some part of project and you want to be notified on changing some file or directory in there. It can be just a button on top of files tree. One more table (project_id, watch_path, user_id), one more notification type.
PS. Although it can be easily implemented with WebHook in a separate application if someone needs it but having it built-in is a more powerful solution.