In Drupal 7 one could store every random thing not having its own table with
variable_set and retrieved it with
variable_get and life was simple. There was the
$conf variable in settings.php which allowed an easy override. This worked well... until you wanted to copy variables from one server to the other. Or translate them. In Drupal 8, most variables have moved to
config() and are still overrideable by
$conf. There are a few exceptions.