Calibration is a must with alcohol breathalysers. The perfect analogy is a clock that needs readjusting after a long period of time, so does a breathalyser.
After many tests being performed (usually hundreds) the alcohol sensor will need to be changed, or adjusted back to working specifications.
Breathalysers are very sensitive instruments, as the cells are exposed to alcohol over time (especially high levels of alcohol), the accuracy can drift and give inaccurate results. This is why buying a unit that has calibration capabilities is a must!