A Board of Directors has three primary "big picture" responsibilities. The first is assuring that the organization delivers on its Mission. Focus on the mission is key to its success. Too many times a Board of Directors loses sight of the organization's mission or they allow the organization to drift away from the mission. (Commonly called "mission drift.") Many individuals join a Board for the wrong reasons which can result in neglect of the mission. Joining a Board of Directors for political reasons or for one's self-esteem can also destroy an organization.
A good example is discussed in an article in the Wall Street Journal: Clueless at the Corcoran