Leaks may have emails that are defunct, belong to dormant accounts nobody reads, throwaway accounts like mailinator.com, honeypots, etc. This confirms the account belongs to a real human actively reading the email.

You're right that they don't have value but there's still plenty of idiots out there buying and selling useless lists and while they're worthless they have a cost to us as recipients since we have to mark them as spam and delete them.