[jrockway]

The attack is that anyone can make an iMessage account and pretend to be your friend ("new phone who this"); this feature is how you prevent that.

[varenc]

Agreed with the sibling comment. To quote Apple, this feature can "detect sophisticated threats against iMessage servers". Essentially it's to protect against state-sponsored attacks MITMing you. Doing that also probably requires the attackers also have access to some root CA private keys so it's a very small pool.

[dannyw]

I don’t think that’s the attack this feature aims to prevent.

Rather, it aims to prevent someone who compromised iMessage infrastructure, from pulling a dodgy around keys.