From Wiki (yes I know), but this does make sense:
People working with
sensitive information are commonly required to sign a statement to the effect that they agree to abide by the restrictions of the Official Secrets Act. This is popularly referred to as "signing the Official Secrets Act." Signing this has no effect on which actions are legal, as the act is a
law, not a
contract, and individuals are bound by it whether or not they have signed it. Signing it is intended more as a reminder to the person that they are under such obligations. To this end, it is common to sign this statement both before and after a period of employment that involves access to secrets.