This read-only value, carried in a trap, is a generic numeric field whose specific meaning depends on the type of event that triggered the notification. An admin cares because it's the actual measured reading, such as a temperature or humidity value, attached to the alarm event, letting the admin see the value that crossed a threshold without polling the device separately. For example, for a Temperature High trap this might carry 95, meaning the sensor read 95 degrees when the alarm fired.