Some people find this file useful for mocking up apps while others use it to concept ideas or create custom interface elements that work harmoniously with those native to iOS. Whatever your use, we hope you make good stuff with it.
New to designing apps? You may want to get acquainted with some of the support documentation that Apple has put together over at developer.apple.com (login required). PDFs like the iOS HIG, UI Transition Guide, and UIKit Catalog will help you become familiar with how and when to use these elements.