It's a great question, that so many companies get wrong.
Peter Drucker, deceased legendary management author since the '40s, has the most notable insights on the subject:
A mission statement has to be operational, otherwise it's just good
intentions. A mission statement has to focus on what the institution
really tries to do and then do it so that everybody in the
organization can say, This is my contribution to the goal.
So you need three things: opportunities, competence, and commitment.
Every mission statement, believe me, has to reflect all three or it
will fall down on what is its ultimate goal, its ultimate purpose and
final test. It will not mobilize the human resources of the
organization for getting the right things done.
"It's our mission to give assurance to the afflicted" --hospital
"To help girls grow into proud, self-confident, and self-respecting
young women" --Girl Scouts
"Make citizens out of the rejected" --Salvation Army
"Make gentlemen out of savages." --Arnold of Rugby, the greatest
English educator of the nineteenth century, who created the english
"Be the informed and responsible buyer for the American Family" --
*Drucker advocates, "an organization's mission statement should fit on a T-shirt". Also, It's funny to hear Guy Kawasaki talk about the common procedure for creating a common mission statement. 'A company takes it's leadership to a retreat where they do trust falls and ropes courses with people they don't like, and develop a mission statement based around everyone agreeing to broad watered down nice sounding statements.'