Why you should calibrate and service your insulating gas measurement device annually, in simplicity is to assure measurement accuracy. To measure gas concentration, like argon or krypton, of an IGU requires well maintained equipment. After a year the device accuracy can change. Also, there are software updates and other parts that might need maintenance. In service the device is tested with short an long tests to guarantee the functionality.
Without calibration the accuracy is impacted
For example, when the device has not been services for the last 12 to 18 months, device’s measurement accuracy can reduce by 50%. See here the comparison of the device’s measurement accuracy before and after the device has undergone appropriate calibration and maintenance service.