Neat. It sounds like this extra step prevents a situation where, for example, a dev's server-side database gets hacked and the users' relay e-mail addresses are exposed.
The attacker wouldn't even be able to send e-mail messages to the users. He'd also need to compromise the registered domain's mailservers, or their DNS servers (to modify the SPF records), or their Apple dev account to add their own registered domain.
"In order to send email messages through the relay service to the users’ personal inboxes, you will need to register your outbound email domains. All registered domains must create Sender Policy Framework (SPF) DNS TXT records in order to transit Apple's private mail relay. You can register up to 10 domains and communication emails."
https://help.apple.com/developer-account/#/devf822fb8fc