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.
With the internet all abuzz about Apple's new Swift programming language, it seems we've forgotten what language design is about. While Swift may be simpler and more elegant than other languages, it also lacks some very important parts of a programming language - parts that make or break long-term support of the community.