To see why communication may be an integral part of good monetary policymaking, recall that the Federal Reserve directly controls only a single short-term interest rate, the overnight federal funds rate. Relative to the enormous size of global financial markets, the market for federal funds - the market in which commercial banks borrow and lend reserves on a short-term basis - is insignificant.
Control of the federal funds rate is therefore useful only to the extent that it can be used as a lever to influence more important asset prices and  BIS Review 2/2004  1  yields - stock prices, government and corporate bond yields, mortgage rates - which in turn allow the Fed to affect the overall course of the economy.