Composer is Drupal 8's Depencency

Google for 'Drupal 8 dependency'. You will find nothing relevant. Drupal 8 does not have dependencies. It's a self-contained CMS. You can even go and download Drupal 8 from the official site in a zip or tar.gz file. Right? Wrong. Drupal 8 has an undeclared dependency, Composer. And the fact that it is undeclared can cost an unsuspecting developer many a nerve. You can install and build a Drupal 8 site from an archive or git repo pull, but very soon you will discover, that there are many modules that declare external libraries as dependencies in their composer.json file. And as soon as you realize that, you realize that you will need to rebuild your site from the scratch with composer. Simply because composer can't handle your site and it's dependencies correctly unless the whole Drupal site was created with it. Voila. I have had issues with this myself and heard lots of complaints form other devs, wondering, why they end up in a situation when Composer seems to either service their libraries OR the Drupal code base, but not both at the same time. Hope you find this helpful.