Thank you for the opportunity to help you with your question!
A nonprofits mission statement is, arguably, even more important than a for-profit or agency's mission statements are to them. The reason is that the mission statement is the core of what the nonprofit does. If a for-profit or agency deviated a little from its mission its possible that not a lot would happen; however, if a non profit strays, they could lose the entire company. A non-profit exists for a very specific purpose and the board and executive director must have a clear focus on that direction.
Look at Good will's mission statement:
Goodwill works to enhance the dignity and
quality of life of individuals and families by strengthening
communities, eliminating barriers to opportunity, and helping people in
need reach their full potential through learning and the power of work.
If the CEO of Goodwill were asked to begin to start manufacturing automobiles, he would have to ask if this aligns with Goodwill's mission. It does not fit any of the criteria so he would reject doing that particular thing, whereas a for-profit might get away with doing this as long as they made money. As the volunteers and employees work for Goodwill, each one of them must align his or her efforts to achieving the overall mission of the organization. Every non-profit is like that. The entire focus of a nonprofit is on its mission.