What is ServiceRegistry in Hibernate 4??

Configuration config=new Configuration();
config.configure();
ServiceRegistry registry=new ServiceRegistryBuilder().applySettings(config.getP roperties()).buildServiceRegistry();
SessionFactory factory=config.buildSessionFactory(registry);

Why be use ServiceRegistry reference for Creating SessionFactory object in Hibernate 4?
But in Hibernate 3 be can create SessionFactory object without ServiceRegistry?

SessionFactory factory=config.buildSessionFactory();

What is technical benefit of deprecate "buildSessionFactory(registry)" method??