Use of volatile without synchronized for lazy singleton in Android Studio project template "Login Activity"

Question

I am learning Android and have been looking through the base project templates provided by Android Studio 3.4.1. The one called "Login Activity" (accessible via New Project...) sets up an implementation with a Repository class in LoginRepository.java

    public class LoginRepository {

        private static volatile LoginRepository instance;

        private LoginDataSource dataSource;

        private LoggedInUser user = null;

        // private constructor : singleton access
        private LoginRepository(LoginDataSource dataSource) {
            this.dataSource = dataSource;
        }

        public static LoginRepository getInstance(LoginDataSource dataSource) {
            if (instance == null) {
                instance = new LoginRepository(dataSource);
            }

            return instance;
        }

        // other methods excluded for simplicity
    }
}

link to android.com description
link to another question with more of the app code

The use of volatile suggests they want thread-safety (although I'm not sure why since it appears to be a single-threaded app).

1. If thread-safety is needed then why isn't there a synchronized in the getInstance method?

2. If thread-safety is not needed, why is volatile used?

I understand there could be a use of volatile without synchronized in such multi-threaded cases as this other question but in our case we are making a singleton.

Thanks