A public event is one you wish to share with other forum members. For example you wish to announce a real life event that other members can attend, or that you wish other users to know.
By creating a public event any users within the usergroups you allow to view the event will be able to read the information you have provided. It is also possible if you have the correct permissions to create a thread from the event.
By creating a thread from the event you will be able to receive feedback on the forums directly related to your public event.