In addition to attracting buyers from other smartphone platforms, mostly Android, Apple relies in part on a shrinking pool of first-time smartphone buyers. Yet at least in the U.S., first-time smartphone buyers, either those coming from a basic flip phone or true first-time mobile phone owners, choose Android phones by a large margin.
A natural place to start out with Swift and Xcode is to build Single View Applications. This is a great template that Xcode provides and is a great starting point for almost every project. But as you grow your applications you begin to need more than one screen or view and hence more than one view controller.
Custom View Controller Transition animations were introduced in iOS7 and they provide you with the ability to customize every aspect of how view controllers animate between each other. This means you are no longer stuck with just the stock animations that are provided in Xcode and you can give your App its own unique feel.