Because only if it is widely accepted that the central bank will keep inflation under control, will businesses and individuals not respond immediately to signs of strong demand pressures by seeking to raise prices and wages. The recent U.S. experience attests to this.
Indeed, strong credibility has been a key factor that has enabled the Fed to take account of the possible changes in economic structure that I mentioned earlier and to steer the U.S. economy successfully towards levels of aggregate demand and employment that in the past would have been thought incompatible with maintaining low inflation.