It keeps strays in the flock. To word it differently: 'You must live up to the popular code if you believe in it; but if you don't believe in it, then you MUST live up to it!
The quote highlights the importance of adhering to societal norms and expectations, suggesting that those who believe in a particular code should uphold it, while individuals who do not believe still have a responsibility to act according to those standards. This duality emphasizes the role of social pressure in shaping behavior, reinforcing the idea that conformity can maintain cohesion within a community.