I'm seeing the following error message any time I try to use Drush to install a module:

> This codebase is assembled with Composer instead of Drush. Use `composer update` and `composer require` instead of `drush pm-updatecode` and `drush pm-download`. You may override this error by using the --pm-force option.

Why does Drush generate that error message? The website was not built using Composer (though there are other Composer-based Drupal project on the same web server).

FWIW, using the `--pm-force` option works just fine. I would just understand the reason of that error message.