The main reasons for this are to ensure the reliability of the instrument, that it can be trusted. To determine the accuracy of the instrument and to ensure the readings are consistent with other measurements.
Considering the costs arising from legal action for damages that might be filed against you, the expenses for proper calibration and adjustment are minor in comparison. Companies who take their responsibility seriously and wish to establish long-term business relationships therefore need to calibrate their equipment regularly. It could also void your warranty if your instrument is not calibrated.