One important reason for consultation and communication between the Federal Reserve and both the executive and legislative branches is the desirability of effective coordination of monetary and fiscal policies. The executive and legislative branches collectively set fiscal policy, while the central bank sets monetary policy. The appropriate monetary policy must give substantial weight to prevailing and expected fiscal policies.
To a lesser degree, the same principle is at work in the formation of fiscal policies. I say to a lesser extent because I believe fiscal policies since the early 1980s have been set more on the basis of longer-run considerations - such as promoting growth - than for short-run stabilisation purposes.