As equipment ages and is subjected to daily wear and tear (temperature and humidity changes, vibration, mis-handling, etc), equipment performance gradually degrades. At some point the equipment will become unfit for the purpose that it was intended. When this happens, your test results become unreliable (without you realising) causing possible safety issues, product design faults, and lower production quality.
It is recommended that employers and employees should ensure that all test equipment is calibrated at regular intervals (multimeters, PAT and RCD/LOOP testers, Insulation testers etc). This ensures that the equipment being used is safe and reliable for the wellbeing of yourself, your co-workers, and your customers using your products and services.