A central bank with a mandate to keep inflation low and stable will generally want to tighten policy to cool the economy whenever unemployment dips below the natural rate and loosen policy to promote faster growth whenever it rises above the natural rate. The appropriate response of the Fed in this case is to tighten policy to prevent inflation from rising. In this case, there is no need for the Fed to tighten policy. Doing so would needlessly slow the economy and lead to an increase in unemployment. One approach says it shouldn't.
Certainty equivalence allows the policymaking process to take place in two stages.
This approach provides a clean separation between the preparation of forecasts and the making of decisions. In many situations, the costs of making a mistake are not symmetric. Consider your actions when you have a plane to catch and the flight is scheduled to depart at 6 p.m. The time it takes to get to the airport is uncertain because of possible traffic delays. Arriving twenty minutes early does not have the same consequences as arriving even five minutes late.