Efficiency and simplicity. Makes the tax system easier to understand and administer.
Originally conceived as a method of collecting money for pensions, this is not a hypothecated tax and now serves no distinct purpose. By scrapping it and increasing income tax we could pay less and the govt would collect the same by removing a tier of civil servants responsible for administering it and losing the associated cost.
Benefits could be linked to payment of income tax (instead of NI) if desired.