It was originally intended as an insurance. It's just that when the government gets the ability to manage a source of income, the temptation to stop hypothecating it and just dump it into general funds - so that it can subsequently be increased without needing to spend the additional income on the thing it was originally intended for - is too great. Exactly the same thing happened with the Road Fund Licence - it was originally intended as a means of funding the provision of roads and other forms of transport, but the link between income and transport expenditure was broken and it got renamed to be an excise duty instead.
Mark