In most developed countries employers deduct the tax from employees' pay each month and then...

The tax is collected earlier than systems that assess earnings at the end of the year.

The payment of tax is easier as the tax is deducted before the net salary is paid to the employee.

Most of the administration costs are borne by the employees.

The responsibility for the tax calculations rests with the employer and therefore there is less chance of mistakes being made.

There is less chance of interest and penalties being levied on the employee by the tax authorities.