I've found these kinds of clients useful because I can remotely wake-up or sleep computers, track how long they are turned on for, and automatically pause my lights and music when my webcam turns on.
This is so cool, first MQTT-based sensor I've set up. Already had a broker set up with HA, but how can HA automatically discover which topic to listen to, know the vendor name and how to interpret all the data?
There's a set of special topics under homeassistant/ that devices also publish to that describe what each topic does and how HA should present it. HA will subscribe to everything under that root topic to discover all your MQTT devices.