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.