The general options list a lot of options. Think of Skype but limited to people who use the app. These calls are free as they are made within the app. Twitch's Desktop application for windows supports text messaging, but also voice and video calls. If you enable this feature, and it is only useful if you do stream on Twitch, then you are assigning Twitch channel roles automatically to server members as they join, and it lets anyone watch the stream within the app. You may also sync your server with your (Twitch) stream. This includes setting the server to public, defining and modifying roles, adding members, banning users, planning events, and more. If you create a server, you get access to the admin panel to customize it.
