I was about to ask the same, I assume it's for recovery in case you mess up the OS, without data accces before first unlock, you are stuck with a brick unless you can boot and unlock it.

Bur I'm not sure I got it right.

My question would be more, what is the rest to select charge only when locked? Is there a risk that the device cannot be recovered? Or is there always a possibilty to reinstall from scratch in case of serious issues that prevent booting?

This would benefit a user who uses a dock on startup, or has an extremely long passphrase for the owner profile that cannot be brute forced. If they have a keyboard or another device responsible for typing the key, they can use that instead.

Some USB keys may have features to store a password in them and type them when used in a certain way. Since the phone only would accept the accessory in a BFU state there is not much added risk.

We have had a few OEMs who have wanted to work with us but so far none has been able nor willing to follow through on what we want with security requirements. We aren't going to support a device that is less secure than what we support already and if something were to go wrong on that device we will get the blame for their incompetence or lack of ability to provide such security that we were able to use elsewhere.

https://grapheneos.org/faq#future-devices

Other sane OEMs do anticonsumer practices, like Samsung with having an eFuse that breaks security features (and the camera in some models) when you want to use another OS. This shouldn't be a thing, but sadly it is.

We still want more devices and are still looking for OEMs. Buying a device secondhand will stop you giving money to Google as another means. DivestOS is an option for non-Pixel devices who use a small amount but not all of our enhancements, but it's mainly used for harm-reduction for insecure end of life devices.