Basically, the plugin prevents commenters from using the name or email address of any existing user in the database. So, if somebody tries to comment as somebody else on the site, then it won't let them. They will need to log in, and they can't do that if they're not actually that person.
A simple and obvious optimization which is useful if you allow open commenting. If you don't allow anonymous and open commenting, then this plugin really has nothing to offer.
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