A calibration is a matter of qualifying the sensor-under-test. Only by knowing the limitations of the sensor it is possible to trust the measurements and optimize the process control. In other words, a temperature reading is only valid if the uncertainty value can be accounted for.
For example: 60°C ±2°C
In this case the uncertainty is ±2°C. This means that a temperature reading of 60°C could have any value between 58°C and 62°C. The lower the uncertainty, the more accurate are the measurements. The more accurate the measurements are the closer can you go to the limit of your process to manufacture your product in the most efficient way. If you do not determine the uncertainties the consequences could be an expensive waste of energy and material produced.