For those unaware about what the factory images are for, they are used when you accidentally brick your phone and you have no way to get it up and running. You take the disk images and flash them directly to your device, in this case the Samsung GALAXY Nexus, and your phone is running with factory settings. Most smartphone owners never have to worry about the factory image and if you don't plan on hacking your phone, or doing anything that could turn it into a brick, you probably will never need to worry about this.
On the other hand, knowing that the factory image has been updated means that the phone itself has been updated and that is something that you always want to know about.
source: Google via AndroidCentral