The point of the "Add New Campaigns to Opt-In lists" feature (which is one that I originally requested a long time back) was so that authors did not have to find and edit the opt-in module every time they made a new newsletter in order to make it active, and thus allow the subscribe and unsubscribe features to work. The idea was that it should be seamless upon creating any new newsletter.
The help tip also makes the same point "Check this option to have new Campaigns automatically added to the Opt-In modules so that users can subscribe right away."
If enabling that setting doesn't make them active automatically, then it isn't doing anything at all, because new campaigns were and are always added to the left side regardless.
This did work initially but hasn't for quite some time now.