Without seeing the circuit diagram, the Field CB allows you to switch off the alternation by removing the magnetic field. If for example you had an alternator failure it could place an unwanted load on the battery bus and cause a fire therefore, there needs to be a means of isolating the alternator from the bus. Because of the higher current this would normally be a CB that activates a contactor. In larger aircraft you may have a means of shearing the alternator drive as a further form of protection.