I'm using D7 and 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.