The original purpose of the registration was to inform the TO about who is coming to the tournament. The reality is that TOs tend to use Facebook events for that kind of information. The minor benefit of registering is that it gets into your personal list of tournaments and you will be notified visually when it becomes available for claiming the tournament spot. (Maybe I will add email notifications later on)
Yes, TO adding notes to registrations is a planned feature. Also registering with decklists (only visible for TO) will be a feature as well.
Anyways, thx for raising questions. Feel free to add ideas here. Also if you are familiar with Github, you can add feature requests, bugs here: https://github.com/madarasz/always-be-running/issues