From a forum discussion. Steve deserves credit.
[[https://forum.netgate.com/topic/180107]]

Offline Config Mode

• Provide ability to set configurations while no internet or DNS is available. This is pursuant to bug #12141
• 23.01 seems to have reverted a little when no WAN is present, while certainly better than 2.6.0 is still quite frustrating
• There are use-cases where not having any internet but wanting to even generate a config to load on another firewall with say a static WAN IP to make onsite install easier
• When diagnosing a firewall where possibly the internet is down, and even the resolved issue is a bit too much wait especially under a stressful condition
• Have an option that shows in the wizard as a checkbox
• Have an option in the CLI, like hit "15 for Offline Config mode" before you log in. (Steve W notes "a simple php shell script in the CLI")
• Have an option under the Diagnostic tab, to allow you to fix a wan or other routing issue without the web interface tripping allover itself