Why are central bank statements so important? - Disclosing private information

It used to be easier to assess monetary policy; follow the rate change or surprise and work through the dynamics on markets. With extended low rates and a zero bound, the flexibility of central banks to adjust rates is limited, so investors have to look at small changes in the forward guidance or statements to make an assessment. Everything rides on the nuance of writing, forward guidance, comments by Fed officials and summary of economic projections (SEP). 

In this case, investors are looking for small changes in statements to assess when policy will change and provide some insight on the central bank's assessment of economic growth. If there is a statement that tells us something on expected growth that can be useful for changing growth assessments, investors may react positively on negative rate news. This is especially true if the central bank is a better forecaster than private economists. Their economic assessment is also more important about when or how the central bank will react since they have inside information on the central bank reaction function.  

An immediate reaction of the stock and bond markets may be sizable and last much longer if there is an implicit view on economic growth. Investors have to look at the disconnect between stock and bond markets to give insight on what the Fed is thinking. See "Deconstructing Monetary Policy Surprises— The Role of Information Shocks" in the American Economic Journal: Macroeconomics

Investors may not like reading Fed tea leaves, but it is all the more important when policy changes and choices are less frequent and rate signals are constrained. Slight changes in commentary will have important changes in distributions and in relative reaction across asset classes. Forward guidance however poor is all we have from a constrained Fed.