@hi short answer is "because Mark Kettenis told me to" :)
From what I understood, it grab two parts from the u-boot image, the boot-loader and the firmware. I'd rather have grabed file and do regular copy/paste on a filesystem but that doesn't seem to work this way.

@hi @joel Oh, I've seen this pattern before! It writes u-boot to the disk but does not overwrite the partition table that's already there. If you didn't care then a single dd does everything, but the above stops bytes 444–511 being written.