Working offline is actually quite important. The download size is over 4 GB for the combo updater, even just for a point release. In this case, the Internet connection was fast enough, but in many places in the world it would take hours longer to download that (10 MBps at my parents' house, 250 KBps in Kiribati where I travelled last year).
It's also good to not need to connect to the App Store, with all the 2FA involved. That would associate my Apple account with that computer, and save my password in the keychain. (Yes, I know I can delete it, but I want to leave the migration running overnight and just tell the user to reboot when it's done).