The short answer is "because the law says so". In England and Wales state education is provided through the operation of a partnership between central and local government, voluntary interests (e.g. the Churches), school governing bodies, headteachers and the staff.
The law requires each school, or federation of schools, to have a governing body. The governing body is made up of people who represent many interested parties (including parents, teachers, the Local Authority, the Church and the community) and its task is to oversee the running of the school.
Essentially the role of governors is, in essence, to represent the public in the running of schools, whose quality affects all of us, and to bring to that task fresh perspectives from ordinary life.
The law states what governing bodies are required to do and the rules by which they must work, you will find these in A Guide to the Law for School Governors which is available to view at www.education.gov.uk
The roles and responsibilities of governing bodies
A strategic role means the governors decide what they want the school to achieve and set the framework for getting there (the strategic framework). This means:
The main requirements are:
It is, indeed, a responsible job and you may feel nervous. Remember, however, that you have been appointed, elected or co-opted by people who had confidence that you could do the job.
Effective schools a positive relationship should exist between governors, headteacher and staff. The basis of this relationship is communication, understanding, support and teamwork. It is a good idea for the new governor to show an interest in the things the school is proud of. Shared enthusiasm is a sound basis for partnership.