An alternative, and more modern, rationale for the once-a-year theatre of Budget day is provided by Keynesianism. According to an extreme version of that doctrine, the private sector is inherently unstable and the expenditure and tax revenues of the state must be varied annually to offset that instability and keep the economy growing steadily. According to this narrative, annual Budgets produce a "judgment" — technically, a change in the cyclically adjusted public sector financial deficit — which constitutes the cleanest measure of "fiscal policy". The stance of fiscal policy is then taken to be a key determining influence on the economy over the next 12 months, when another Budget judgment emerges from the Treasury. That subsequent judgment influences the economy over the following 12 months, and so on. 

At one time — say, in the 1950s or 1960s — many well-informed people thought that the annual Budget palaver could be justified on Keynesian grounds. But in the last 30 years fiscal policy has played second fiddle to monetary policy, with the Monetary Policy Committee at the Bank of England now taking the most vital decisions on the price and quantity of money, and hence on macroeconomic policy broadly understood. The demotion of fiscal policy in macroeconomic policy ought to have been accompanied by a de-emphasising of the Budget in the political calendar, but that does not seem to have happened. 

In short, there is no compelling reason for an annual tax-changing event. The huge amount of fuss and bother that attends this event tells us that something wrong with how our country is organised. The centrality of "the Budget" in the British political system arises from the inability of modern governments to deliver a stable, predictable and sustainable financial framework in which people can make long-term plans for their lives and businesses.