I'm working on a standard Action Extension in my iOS app and the Xcode template contains the lines:
@IBAction func done() {
    // Return any edited content to the host app.
    // This template doesn't do anything, so we just echo the passed in items.
    self.extensionContext!.completeRequest(returningItems: self.extensionContext!.inputItems, completionHandler: nil)
}
Here is the documentation for completeRequest.
Questions
- What is the purpose of passing 
returningItemstocompleteRequest? - Do apps actually receive edited content from an action extension?
 - If yes, where can I find the API on the receiving end?
 - What are the consequences for me to pass an empty array?