Many tests require that a user logs into the site. With the blackbox driver, all user creation and login would have to take place via the user interface, which quickly becomes tedious and time consuming. You can use the Drush driver to add users, reset passwords, and log in by following the steps below, again, without having to write custom PHP. You can also do this with the Drupal API driver. The main advantage of the Drush driver is that it can work when your tests run on a different server than the site being tested.