What is the necessity of having a shared instance of appDelegate?

I found some come that I’m having trouble to understand

let appDelegate = UIApplication.shared.delegate as! AppDelegate

which is used in a "Loader" class

   let appDelegate = UIApplication.shared.delegate as! AppDelegate
   let storyboard = UIStoryboard(name: "Main", bundle: nil)
   let root = storyboard.instantiateViewController(withIdentifier: "RootView")
   root.modalPresentationStyle = .fullScreen
   self.appDelegate.window = UIWindow(frame: UIScreen.main.bounds)
   self.appDelegate.window?.rootViewController = root
   self.appDelegate.window?.makeKeyAndVisible()

but the "same code" can be found in the "AppDelegate" class

    if let windowScene = scene as? UIWindowScene {
           let storyboard = UIStoryboard(name: "Main", bundle: nil)
           var vc : UIViewController = storyboard.instantiateViewController(withIdentifier: "RootView") // CHECK (???)
           self.window = UIWindow(windowScene: windowScene)
           self.window?.rootViewController = vc
           self.window?.makeKeyAndVisible()
           }

I’ve realized that it can be used to access the AppDelegate methods and variables from other classes, but I’m failing to grasp the potential of this approach and the reason it is implemented

>Solution :

You have a single instance of the application running, that is an instance of UIApplication. And this application is assigned a delegate, which you implement. Since you can get the running application instance with UIApplication.shared, you can also query its delegate and you thus have a "singleton" of your application delegate as a by-product.

People often end up packing a lot of code into their app delegates since it can be conveniently queried everywhere in your app and it usually "knows" or has direct or indirect access to most pieces of the app.

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