我好不容易在 App Programming Guide for iOS 這份文件的 The App Life Cycle 這的部分找到對於 AppDelegate 的描述。

App delegate object

The app delegate is the heart of your custom code. This object works in tandem with the UIApplication object to handle app initialization, state transitions, and many high-level app events. This object is also the only one guaranteed to be present in every app, so it is often used to set up the app’s initial data structures.

這樣就可以解釋為什麼 UIApplicationDelegate 裡的方法要在 AppDelegate 裡面實作了。