In the UK, there's a type of income tax called "Employer’s National Insurance". This is calculated just like the other forms of income tax and is paid by the employer directly, but the employee never sees this on their payslip and it is not part of their negotiated salary.
(For example, say you're an employer and you've budgeted £1000/month for a potential employee's salary. You'd have to offer that person £878.73/month salary to bring the amount you'd actually be paying to the £1000 you've budgeted for.)
What is the rationale for having this separate income tax on top of the normal salary level?
Note that I'm particularly interested in the rationale behind the portion of the tax that doesn't appear on the payslip, rather than the virtues of national insurance itself.