MQTT what is the purpose or usage of Last Will Testament?

Question

I\'m surely missing something about how the whole MQTT protocol works, as I can\'t grasp the usage pattern of Last Will Testament messages: what\'s their purpose?
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Answer 1

Just because a device is not publishing does not mean it is not online or there is a network problem.

Take for example a sensor that monitors a value that only changes very infrequently, good design says that the sensor should only publish the changes to help reduce bandwidth usage as periodically publishing the same value is wasteful. If the value is published as a retained value then any new subscriber will always get the current value without having to wait for the sensor value to change and it publish again.

In this case the LWT is used to published when the sensor fails (or there is a network problem) so we know of the problem as soon at the client keep alive times out.