allow communication between Elm and JavaScript. page 
Ports are probably most commonly used for WebSockets and localStorage. Let's focus on the WebSockets example.
Ports as the basis of an Object-Capability Model on the web page
Ports can be viewed as a way to expose limited capabilities (in the object-capability model sense) to other actors in the system. This can either be a weak capability system, where the ports are merely used as a convenient model within a particular origin, or as a strong capability model, where they are provided by one origin provider as the only mechanism by which another origin consumer can effect change in or obtain information from provider.
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Using message channels and MessagePort objects, however, all of these problems can go away. When the game tells the social site that it wants to add a contact, the social site can ask the contacts provider not for it to add a contact, but for the capability to add a single contact. The contacts provider then creates a pair of MessagePort objects, and sends one of them back to the social site, who forwards it on to the game. The game and the contacts provider then have a direct connection, and the contacts provider knows to only honor a single "add contact" request, nothing else. In other words, the game has been granted the capability to add a single contact.
See Runtime