When using injection, application code would generally not need to interact with the implementation classes. For example, it would receive a Datastore or Storage instance directly and never need to interact with any of the factory or ServiceOptions classes e.g.

class MyApp {
  @Resource Datastore dataset;
  @Inject Bucket data;
  void doSomething() {
    // just use dataset and data fields ...
  }
}

By separating the API classes that it would use into a separate jar, the user can avoid having a dependency on the implementation itself and on any of the libraries that the implementation happens to use.