The VOC sensor works best if it is placed in a room where it regularly sees clean, fresh air. For example - a bedroom that is regularly ventilated. If there is little difference in VOC readings when the room is ventilated compared to when it is not, it is a good sign that VOC offgassing in this room is low (one important note: the sensor will react to you, so try not to be in the room yourself when using the sensor to evaluate VOC offgassing like this).
The VOC sensor can also be used to indicate air that becomes stale from too many people. The sensor detects VOCs in people's breath (and perfume and other smells), so constantly high levels of VOCs in your bedroom at night might indicate that you should ventilate better while you are sleeping.
It can also be useful as guidance to ensure your air is clean after cooking or cleaning. Typically, VOC levels will rise during cooking and cleaning, and you should ensure good ventilation during and after to keep your air healthy. The VOC sensor offers one way of assessing how much you should ventilate to get back to the air quality you had before you started cooking or cleaning.