Mail Server Thing Delete Messages

I can't perceive a difference in the "withDelete" boolean parameter of MailServer.ReadMessages().

withDelete : false - Only newest, unread by MailThing, emails are show. Next ReadMessages() call will show no messages if no new messages have come in. Messages remain on actual server, (gmail)

withDelete : true - Same exact behavior

I know the messages remaining on gmail server is based on gmail settings. But regardless, what does that withDelete mean?

Either way old messages can't be seen, essentially messages can only be retrieved once.