Measurement errors can be expensive.
Without this certificate you cannot trust your density measurement, and errors will compromise your quality control. A surprising number of hydrometers lose mass between calibration, almost always due to damage to the bulb. So regular calibration is important. A UKAS calibration certificate is supplied with each hydrometer, showing calibration corrections. To maintain accuracy it is recommended that hydrometers used at ambient temperatures are calibrated when new, and then every two years.