Updating an existing Drupal 8 installation to a Composer installation is actually pretty easy. There are already a few scripts out there that will help you achieve this. By following this step to step guide you'll successfully update your Drupal 8
In some use cases you want to copy the configuration of a Drupal 8 website into another Drupal 8 website. For example when you are working on a multisite and you want to create a copy of an existing site.