Before any storage medium (e.g. floppy disk, hard disk or CD) can be used, it must get a filesystem (DOS speak: get formatted). The filesystem is responsible for organizing and incorporating the files that should be stored on the medium.
The usual utilities for creating filesystems on hard disk partitions write an empty filesystem onto them, which is then mounted and filled with files by the users as they need it. A writable CD is only writable once so if we wrote an empty filesystem to it, it would get formatted and remain completely empty forever. This is also true for rewritable media as you cannot change arbitrary sectors yet; you must erase their whole content.