title: MQTT what is the purpose or usage of Last Will Testament?
body: <p>I'm surely missing something about how the whole MQTT protocol works, as I can't grasp the usage pattern of <em>Last Will Testament</em> messages: what's their purpose?<br>
  One example I often see is about informing that a device has gone offline. It doesn't make very much sense to me, since it's obvious that if a device isn't publishing any data it may be offline or there could be some network problems.</p>
  
  <p>So, what are some practical usages of the LWT? What was it invented for?</p>