Because such changes in perceptions could be counterproductive, concern about triggering them might discourage a central bank from taking action that otherwise could have been appropriate and beneficial for the economy in the near term. Lack of transparency and lack of credibility, in this sense, could reduce the effectiveness of monetary policy in stabilizing the economy against transitory shocks.
A concrete example of this can be seen in recent developments in the United States. Consider the pickup in inflation that we saw last year, which many economists would attribute at least in part to the temporary effects of increased energy costs.