During a new Mac setup, it asks for the iPhone passcode. Why? Why are they asking me for this? What is it used for?

EDIT: Some have suggested that this is a recent effort to encrypt all of the iCloud data: https://www.reddit.com/r/apple/comments/4b70xy/starting_with_ios_93_apple_will_encrypt_icloud/?st=irfs7s30&amp;sh=2a9995dc
I think that you've selected to use your iCloud account, so by verifying your Passcode with you iphone Apple added a security to be sure that you are the owner of your iCloud Account. This will allow your Mac to synchronise your data from iCloud.