The word Drush is a combination of two words: drupal + Shell.
A shell is a command line tool that lets us manage our website by writing commands.

Why should you use it?

Drush is a Drupal developer's best friend, it makes your life easier.
Instead of hovering around the interface, Drush is an excellent alternative tool for doing various tasks with Drupal using commands. Save yourself from all those page clicks and more!