Why do you calibrate and recalibrate measurement equipment?

Answer 1

Usually, when you measure something that requires calibration, that work is generally considered important either monetarily or socially. So, you want to make sure you are sure because things are riding on your measurements. The short answer is so you don't screw up :)

Answer 2

It is not necessary to calibrate the instrument every time before use, but checking the accuracy before use is important.

Fine measuring instruments like micrometers are sensitive to temperature and impact. These instruments are specifically designed to measure to 2nd or 3rd decimals and if one is dropped or accidentally bumped against a hard surface, it could lose its accuracy and has to be calibrated. Most companies that conform to quality standards have their instruments sent away and calibrated by reputable calibration companies every 6 months to once every year depending on their standard.

Sometimes an artisan might leave the cold instrument on top of his lathe. The lathe is usually hot and could affect the accuracy of the instrument by heating it up.

So, depending on the level of accuracy you are working at, constantly checking the instrument's accuracy ensures a precise and quality workpiece.