This scheme is typically useful for a mailing list where a user mailing to the list has no control over the maintenance of the list itself; in this case the list maintainer would be the owner of the list. For example, the following would cause jd@1company.com to get the error that occurs when someone sends mail to unix-hackers, and sendmail finds the phony user nosuchuser on the list.
owner-x
unix-hackers: jd@company1.com, ed@big.university.edu,nosuchuser, jane@company2.com
owner-unix-hackers: jd@company1.com