`dd` is very useful for backing up the boot sector of a hard drive or other storage device (`dd if=/dev/sda of=boot_sector.bin bs=512 count=1` ) and then later rewriting it (`dd if=boot_sector.bin of=/dev/sda`). It is similarly useful for backing up the headers of encrypted volumes.

`cat` might be able to be twisted into doing that but I wouldn't trust it on the rewriting part.