Switch an existing Drupal 8 project under composer
Composer has become a must for relatively ambitious Drupal 8 projects. Even if it is still possible to initialize a Drupal 8 project with drush or simply by downloading a zip archive, these two methods can become limiting over time. Or at least not to facilitate the installation of new modules with dependencies on third-party libraries.
Another of the reasons I have encountered, why some Drupal 8 projects have not been initiated by Composer, is the lack of Composer support on some shared hosting, even so-called professional ones.