If multiple pages are listening for onMessage events, only the first to call sendResponse() for a particular event will succeed in sending the response. All other responses to that event will be ignored.

When multiple tabs consume your application and you need to have a common gateway for the interaction.

Background pages are the controllers of our application and exist through all the time our application is alive. They can be consumed by any tab at any time as long as the Background Page has registered an event listener

An extension "registers" callbacks to Chrome for a specific situation given you described a specific interface to do so