The traditional argument in favour of a strong autonomous central bank is that the power to spend money should in some way be separated from the power to create money. According to this argument politicians generally have short-term horizons in determining economic objectives. Many governments have accordingly given way to the temptation to reduce interest rates ahead of elections.
This may boost spending and employment in the short term, but ultimately it normally also causes higher inflation over the long term unless the capacity of the economy can meet this higher level of demand. This higher inflation, however, only becomes apparent a couple of years later.