The idea
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.
Why is it important?
Efficiency and simplicity. Makes the tax system easier to understand and administer.