I think the point of this technique is to be able to leave the machine locked on cold boot, but to be able to e.g. unlock it, put it to sleep, and go on vacation; and then, if you need to remotely reboot it, you can reboot it in such a way that it stays unlocked on next boot, rather than reverting to locked.
When it comes to disk encryption, at least in the home, the threat model isn't somebody sitting around in your home finding a way to exfiltrate the currently-unlocked filesystem; It's someone taking the computer or the drive with them and leaving.
In your analogy, the key atop the vault vanishes as soon as the vault is moved from its location (loses power).
That wasn’t what was asked for. The original reason given was to require a password on cold boot, but not require a password when rebooting e.g. for an OS update
