I started using Drupal 8 from early on. Since Drupal 8.2, it has made Composer and undeclared dependency. I realized I could no longer install modules and manage dependencies in a sane way, as well as update my sites, without converting them to use Composer. Converting the sites to use Composer can be hard if you have issues with your site, like missing modules or broken configuration. If your site is healthy, though, following the next 10 steps should not be hard.