For the last two days, Google has been rolling out some bug-fixing updates for Nexus 4, Nexus 7 (2012), Nexus 7 (2013), Nexus 10 and Galaxy Nexus devices. These updates are minor but bring solution for some critical problems present on the Nexus devices, so the company has decided to post the latest Android 4.3 factory images containing these bug-fixes.
Nexus 7 (2013) gets the 4.3 (JSS15Q) based factory image, while other Nexus devices get 4.3 (JWR66Y) based factory images.
If you haven’t received the OTA updates for these newer builds, you can also use these factory images to flash your Nexus device to the latest Android 4.3 build but you might lose all your user data.
Factory images are the best way for the adventurous Android users, who love to flash custom ROMs on daily or weekly basis, to return to the factory version if needed.
Source: Google Developers Website