Drush (drupal shell) is just a command line interface to any given Drupal site, similar to Laravel's Artisan console, or Django's django-admin & manage.py, or Wordpress's wp-cli. Managing a Drupal site is much simpler when you can do certain things from the cli: run cron tasks, cache rebuild, import/export config, enable/disable modules, use the built-in REPL to troubleshoot stuff, not to mention the various commands that modules expose.