Each school's mission should be unique to that school and is based on the needs of their community. The schools that take their vision and mission seriously have a better focus on their students and their needs.
I'm skeptical that it really is important. I think that schools create these things because they think they are supposed to and then they never do anything with them.
However, if a school were to take this seriously, it would be important because it would mean that all the teachers and everyone else would be on the same page. They would all presumably teach with the same goals in mind and they might all teach in the same general way. This would give the students more of a consistent message about what is important and it might make it so they did not have to adjust to different expectations from different teachers.
Not every school is the same. Schools need to be responsive to their community and student population. Some schools are designed with specific instructional programs in mind, such as charter schools. They might have a back to basics focus or an arts-centered focus. However, most schools are just neighborhood schools serving whatever population comes to them. Therefore it is very important that the school develop a vision and mission statement to respond to the needs of the students who attend.
As the instructional leader, it is the principal's responsibility to draft the mission and vision statements and to solicit input from staff, parents and even students. The vision and mission statements are somewhat "living" documents, constantly being tweaked as the school's needs change. Changes in demographics are quite common, and so are changes in instructional focus from one administration to the next. Therefore, a school's vision and mission will change.
Good educational organizations create public statements that express the what the organization understands to be their mission (job) and their vision (how they are going to go about their job and what results they anticipate). These ideas should be grounded in and reflect the philosophy of education that is embraced in that organization. Not all schools operate under the exact idea of what is important to the education of a child or how that education can be delivered. The mission of a fine arts school will be different from the mission of a vocational school. A Montessori school will be different from most public schools. A Catholic school will see that they have a different mission than a public school.
Successful schools have conversations about the mission and the vision of the school with everyone involved in the organization, so this includes the administration and staff, but equally important, it includes the parents and children, as well as the community. By have this open dialogue, everyone involved in the education of the children has an understanding of what the goals and objectives of the school will be. This provides a structure for the teachers in the creation of curriculum; it provides a reference point for parents to have a broad overview of what the school stands for; it provides the administration with the backbone from which to support new and existing programs, and from which to be in a position of evaluation and adaptation. Without vision statements, there is a potential lack of continuity and unity within the organization, and that can lead to incomplete education for the children.