This feature of blacklisting certain email domains already exists in Discourse, but the bonus with this plugin is that you avoid maintaining (and entering) a list of thousands of free email domains.
The good news is that there are only really about 20-30 reliable, popular disposable email services .. so it’s not that much trouble to use the site setting.
But if you want a complete block of hundreds, maybe thousands of potential free email sites, this is certainly a better alternative.