untill now nobody really wanted to clean up its membersbase and kill accounts that were inactive since a year at least. This makes that an enormous number of email- and other services have an enormous number of accounts sometimes with very attractive names that are just dead ducks. The biggest problem to take any action is the danger that if you liberate those accounts it will be possible for the people who were getting the old accounts to get access to other services that were authentificated through that emailaccount
this new header changes all that because it will be indicated in the new header if the emailaddress has changed hands or not and since when. This makes it possible for the other services to compare with their own database to compare the date at which their member became a member. It is not totally waterproof but it is a start.
It also gives numerous services a possibility to destroy accounts that have no sense of staying on their servers and limit also the damage if they have a breach.