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.
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…))
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 …