The History tab displays how many times your device has connected to- and disconnected from the network and at what time(s) it occurred.
Domotz performs a connectivity check (heartbeat) and reports in this section when changes in connectivity have occurred.
Some devices (e.g. mobile) can lose connectivity quite often during normal operation, so such details may not be relevant.
For many other devices, History offers a useful snapshot of the recent history of the device. It can help to visualize a behavior that you may not otherwise notice, and tell you if further investigation is needed.