Like grandparent said: by having a catch-all email address on a domain. Then emails that are being sent to invalid email accounts on that domain end up in that mailbox.
Another commenter in the thread presented how their company "normalizes" gmail addresses by removing "." character and "+..." suffixes, so users don't abuse their system by creating multiple accounts with what is basically the same email. Having a catch-all mailbox allows people to circumvent this "security" measure.
I'm imagining that you actually want to have separate emails pointing to separate mailboxes, and for that case indeed, your solution is better, but for the life of me I can't imagine why anyone would prefer having to check multiple emails, instead of a single one. :D