Answer:
Role
Dissent, then, is a powerful source for developing effective public reasoning, itself necessary for determining the legitimacy of the actions and institutions of a given state as well as the customs and practices of a given society.
when it is necessary
The concept of civil disobedience was one of the cornerstones of the civil rights movement of the 60's. If the government asks you to violate your faith or your conscience, then civil disobedience may be appropriate.
Why is necessary
Civil disobedience provides a check against totalitarianism by showing that citizens won't follow unjust laws and that there are limits to the use of discipline.