From this dialog, you can send an e-mail to all attendees currently registered for the event. This may come in handy in case of changes in the event schedule, when sending additional information about the event, etc.
Detailed information about the Events module can be found in Developer's Guide -> Modules -> Events.
Page url: http://devnet.kentico.com/docs/6_0/contexthelp/index.html?send_e_mail_tab.htm