The App hash is stored in the firmware metadata (you can read that using the BVR https://github.com/spark/binary-version-reader ).
The cloud saves the hash of the firmware running on your device, and publishes an event when that hash changes, signifying that your device is running a new version of firmware. This can be a helpful way to confirm that multiple devices of yours are running a particular version of firmware. It also can be used to ensure the firmware arrived intact on the device before being run.
You could imagine a flash succeeding, but the platform being incorrect for that device, so the device might reject the new firmware. In this case the app hash wouldn’t change, and you could see your device was running a version other than what you sent, etc.
I hope that helps!