Why cdrom is mentioned in grub.cfg and txt.cfg?
In grub.cfg and txt.cfg, its still mentioned
append file=/cdrom/preseed/ubuntu.seed boot=casper initrd=/casper/initrd.lz quiet splash —
The live iso doesn’t fit a cd for quite a long time. Once you install the distro, there is a empty folder cdrom, which is just there.
cdrom here is just a default path which debian-installer picks when preseeding. You can change it to a url but then you also need to change other syslinux/isolinux config files. Also I think you can still create a bootable dvd with it and dvd mounts as cdrom.
I find you can boot the live iso without the
part and then install the iso as (1) we don’t use the Debian Installer, and also that we use (2) only ubiquity and that points to casper.
Ubiquity uses debconf in the backend, just like debian-installer does. The preseed data is applied during installation from that file, if you install without it you will be missing some defaults that are defined in there. The /cdrom folder is simply still used as a mountpoint during install, no matter if you install from cdrom, dvd or usb key (also note that there are flavours using the same installer with different branding that might still fit on a CD (lubuntu, ubuntu mate, ubuntu…))
What data might be missing, if there is no preseed file?
You will have to check the content of the file on the install media, it is years ago that I touched d-i/ubiquity last, but there are a few configuration defaults in it … IIRC there were things like apt-setup defaults, debconf priority (to make sure packages do not ask unnecesary questions on the installed system) tasksel defaults for picking the right task (ubuntu-desktop) and packages to not (accidentially) uninstall on cleanup at the end of the install process …