You may not know exactly yet what the app does but you can clearly see which screens it has and how they are related. That is the power of using storyboards.
If you have an app with many different screens then storyboards can help reduce the amount of glue code you have to write to go from one screen to the next. Instead of using a separate nib file for each view controller, your app uses a single storyboard that contains the designs of all of these view controllers and the relationships between them.