Applications today are rarely being built without 3rd party libraries and SDKs. There are libraries for integrating remote backends into your app. Libraries for effective image caching and loading. Libraries for gathering analytics and libraries for pushing messages. Libraries that help your users report issues, help you debug and analyze crashes. (Yes, that last one is Bugsee)
It is also a common practice for developers to maintain more than flavor of the application during active development. A debug version of the application may require a new version of the backend server which also might be under development still, it might send analytics data to a different service or it may need to include a helper library that is not necessary in the release version.
This following tutorial describes several options for maintaining different build configurations.