If you're robbed at gunpoint, and they demand you unlock your phone for them, this protection means they would have to kidnap you for at least an hour to then unlock it again. The former happened pretty regularly, but upgrading to kidnapping is a lot riskier, more dangerous, and invites a serious police response.
> Security Delay: Some security actions such as changing your Apple ID password also require you to wait an hour and then perform a second Face ID or Touch ID authentication.
Emphasis mine.
It seems like you have a lot of reasonable questions and concerns about the efficacy of this measure, but Apple have done a pretty good job of addressing them and explaining their rationale in the first few paragraphs of the documentation.
> In the event that your iPhone is stolen, the security delay is designed to prevent a thief from performing critical operations so that you can mark your device as lost and make sure your Apple account is secure. Learn what to do if your iPhone is lost or stolen.