This is interesting on what National Insurance is for - it's from Parliament
https://commonslibrary.parliament.uk/research-briefings/sn04517/
The payment of NICs entitles payers who have met certain conditions to receive contributory benefits. The biggest of these benefits is the State Pension. The link between payments and entitlement to benefits is known as the ‘contributory principle’ and was envisaged by William Beveridge in his report on social security and allied services, written in 1942 and partly implemented in 1948. Beveridge had hoped that contributory benefits, alongside other policies, would leave a residual and diminishing role for means-tested benefits.
However, several subsequent developments have eroded the strength of the contributory principle. For example, the role of means-tested benefits has tended to expand, while working-age contributory benefits have remained at a flat rate not designed to meet full household costs. Pension-age provision has not followed this trend, as the value of the State Pension has significantly increased since the 1970s.
Unlike other taxes, most of the money raised through NICs does not go into the consolidated fund (the government’s bank account), but a separate pot of money, the National Insurance Fund (NIF). Money in the NIF is reserved exclusively for spending on social security benefits, such as the State Pension. A portion of money raised through NICs is allocated to the NHS before the rest is transferred to the NIF.
The NIF works as a pay-as-you-go fund. Therefore, receipts from contributions in one year are spent in the same year for contributory benefits. While there is a link between someone’s record of paying NICs and their entitlement to contributory benefits, there is no direct connection between the amount of NICs they pay into the NIF and the value of contributory benefits they are entitled to claim