Who is it for?
Apple says its system is for the same essential group of people it already protects with Lockdown Mode — that is, “users who face extraordinary digital threats, such as journalists, human rights activists, and members of government.”
What problem does Contact Key Verification solve?
While iMessage chats are end-to-end encrypted, that security relies on a third-party “Key Directory Server” to authorize devices. That makes the Key Directory Server a potential target for criminals and surveillance.
The problem comes in the event a powerful entity manages to compromise the security protection of that server; once they have done so, it becomes possible to intercept or monitor messages, or even enter the conversation. (This could be a particular concern for people in politics, human rights activists, journalists, businesspeople and others.)