To "Control" whether or not someone receives a piece of content when using smart campaigns and the Engagement Engine, Edward Unthank has doen a great job of explaining it here: http://community.marketo.com/MarketoDiscussionDetail?id=90650000000PjghAAC
What you are in essence doing it making people a part of that program. What the Engagement Engine does when looking at what to send next it asks itself whether or not a person is a part of that program and if they are then they will skip to the next content.
The other reason you may use campaigns as opposed to emails is because yes you are right in that you can add additional flows. E.g. You can do scoring or wait steps or add to lists.