In the offline world, once the buy is completed and the ads are sent to the publishers, the work is virtually done. In the online world, once the buy begins, the work has just started.
One of the key components of the digital advertising value proposition is optimization. Because most of the impressions run on a user-to-user basis, online marketers can analyze and make changes throughout the flight in order to drive the highest return on investment (ROI). And because they can, they have to. In fact, it's practically unheard of for a digital campaign to not go through any sort of optimization at some point during its lifecycle.