The arm servers like the Raspberry PI 3 get their operating system by copying an image to a memory card. That image contains a MSDOS compatible partition which contains files that can be edited on e.g. windows. Being able to edit the future rc.conf file would allow us to edit the name of the server, make sure that growfs is enabled, enable ntpd, etc. By renaming the file fter use, we have a track record of the inital config and if neccessary it can be done again. Ideally there should also be a possibility to define a non-root account that is to be created the first time. Several FreeBSD derivatives uses different accounts. It would be nice to be able to log on regardless of what account they have decided on. The definition of the account should be overwritten once it has been created to ensure that the password cannot be recovered after the account has been created.
The rc.conf file always existed only in UFS and it will stay that way. The images generated by the release engineering team ensure that the system will boot and that the user can login either by ssh on using the serial console to modify it according his needs. Thoses images have a 'freebsd' account with the password 'freebsd'.