Serial flashers (not that kind) are already aware of Google’s factory images for Nexus devices, that let you download the full ROM and flash it to your Nexus device. The downside to doing this was you needed an unlocked bootloader, and doing that would wipe your device.
Enter OTA binary image files. These allow you to sideload the image to your device with adb through recovery without having to bother with the bootloader. This means you can upgrade easily without having your data wiped (although it wouldn’t hurt to backup, just in case).
As before, these are only available to Nexus devices for now.