Migration Assistant uses the old Mac, which can unlock its login keychain and allow the keychain items to be copied.
What if the old Mac is dead, stolen, lost? Those are the scenarios in which one recovers on a new machine from a backup.
Migration assistant can also use a TM backup, I’m pretty sure
What if the old Mac is dead, stolen, lost? Those are the scenarios in which one recovers on a new machine from a backup.