The true mark of an effective leder is he/she possesses a clear focus and direction. Successful leaders often have a "Vision" of where they are headed. This "Vision" is used to inspire their people and organization.
This vision is sometimes inspired from within and sometimes uses a team effort to help construct it. Doesn't matter where this vision comes from, it's Leadership that aligns,directs and provides inspiration to follow the organizations goals.
Successful managers/leaders then align their vision with their organizations. They are usually just a small part of the whole. This then allows the use of this "vision" to consruct the mission and goals of his/her individual department. Then each team member on an idnividual basis.
To provide direction is one of the basic tools needed for the modern managers toolkit. I highly recommend reading Stephen Covey's "7 Habits of Highly Effective People" It provides extremely effective information on constructing a mission statement.