Why not just set initial view controller for login screen like this?

Question

I've already read this and this questions. But I didn't understand why the code below is not enough. This just works. Do I need an else statement to set rootViewController for LoginViewController? Isn't it already set by Storyboard?

func application(application: UIApplication, willFinishLaunchingWithOptions launchOptions: [NSObject : AnyObject]?) -> Bool {
    if Settings.loggedIn {
        let mainStoryboard = UIStoryboard(name: "Main", bundle: NSBundle.mainBundle())
        let mainViewController = mainStoryboard.instantiateViewControllerWithIdentifier("MainViewController")
        self.window?.rootViewController = mainViewController
        self.window?.makeKeyAndVisible()
    }

    return true
}

In LoginController, should I set rootController of AppDelegate? Why not performSegue?

@IBAction func login(sender: AnyObject) {
    performSegueWithIdentifier("loginToMainSegue", sender: nil)    
}

Also why not performSegue to logout?

@IBAction func logout(sender: AnyObject) {
    performSegueWithIdentifier("MainToLoginSegue", sender: nil)    
}