It doesn't. The bonus is income just like your wages.
An employer might benefit by reporting the bonuses separately and apply the same tax rate to all the bonus checks. That way all the employees (that earned the same bonus) will have the same dollar amount on their bonus checks. The employer doesn't need to explain to Sally why Sue's check is bigger because Sally has more withholding requested on her W4.
But for the employee, bonus income is grouped with wage income on their tax return.