This is certainly a very important step for Office 365 to Office 365 migrations. It makes sure that emails have the ability to be replied to, even after the Full (Delta) migration has occurred, as they will be mapped to the new place area title, instead of using the http://damienckrah.aioblogs.com/13056485/how-office-365-migration-steps-can-save-you-time-stress-and-money