The Fed's dual mandate is to "maintain price stability" and to "ensure maximum stable employment", both of which are only accomplished through explicit manipulation of the economy. They're an enormously powerful entity, and if they're actually accomplishing their goals, do help keep the economy running smoothly (most specifically, IMO, by improving the confidence of people to make long-forecast economic decisions), but when they get it wrong, they have the potential to cause damage at a scale nearly unrivaled by anyone else.