What is the function of fiscal policy?

Fiscal policy is that a government adjusts its spending level and tax level to influence the country's economic growth. However, government can use subsidies and taxation to target some microeconomics problems, such as externalities. Which policy made by the UK chancellor catches the most attention today? The sugar tax. The purpose of this policy is to reduce the children obese problem. However,  I feel surprised that it can catch so much attention. It will influence the food industry; however, in terms of the UK economy,  the impact is almost zero. I believe that the priority of fiscal policy is to maintain a sustainable budget and a stable economic growth. Maybe we have overestimated the impact of monetary policy and underestimated the fiscal policy. Today we may have reached the limit of what monetary policy can achieve. If we would like more policies to create more positive momentum, these policies will be fiscal policies. Fiscal policy can especially impact on the least productive industries. Removing subsidies or increasing tax could force companies to improve their productivity or leave the market. This could increase unemployment rate in the short term; however, in the longer term, if we can have a faster economic growth rate, there will be more opportunities in the labour market. Given the current low unemployment rate, a short-term cost for a long-term gain should be considered. Fiscal policy does not only have an impact on improving productivity, it can also influence the financial market. The bailout made by the government during the latest financial crisis is an example showing what a government could achieve. More than saving the market in bad times, using taxation could change banks' behaviour as well, thus improving the stability of the banking sector. In conclusion, I believe there is much room left for fiscal policy to influence the economy when some central banks have their hands tied.