Android 4.1.1 source code for AOSP has just been released. It took only a few days for Google to release this source code from the launch of Android 4.4.1 KitKat Update.
Why might you want the source code? For many reasons. Developers want it because they want to edit this source so they can create their own builds and port them to their current devices.
Android 4.4.1 brings many improvements over 4.4 but the biggest one is the better camera performance when taking pictures or videos.