The use by central bank of changes in the supply of money and interest rates is to influence the level of economic activity and achieve the desired economic outcome. The central bank may want to use the monetary policy to either boost economic activity (if the economy is in recession) or perhaps to reduce economic activity (if the economy is growing too fast, causing inflation). The economy could be slowed down by contractionary (or deflationary) monetary policy either in:
1...increasing the level of interest rates
2...reducing the rate of growth of the money supply
Or boost the economy during the downturn by expansionary (or reflationary) monetary policy either in:
1...reducing the level of interest rates
2...allowing the rate of growth of the money supply to increase