Drupal's default methods of handling user names, emails, and registration processing work pretty well out of the box for many web sites. Drupal assumes your users are online, have unique email addresses, and that you want to create a site that grows organically as users find it and register themselves. Drupal out of the box may not work so well for real-world groups of people where the group already exists and consists of specific people who may or may not be online, may or may not have unique email addresses, and may or may not be able or willing to register themselves on your site.