Why?

Maintaining balances used in the laboratory is completely different from maintaining normal scales. Weighing chemicals and other light objects need being performed on balances with high desired accuracy and precision. 

Laboratory technicians, teachers and students won’t be able to obtain accurate weighing results if the balances are not calibrated regularly. 

The accuracy of lab balances and scales become less reliable over time due to normal wear and tear of regular uses and mechanical shocks or hazardous environment. Ignoring maintenance activities will turn measuring into guesswork.