Shivtei Yisrael

We often sit through the haftorah without understanding what it is all about. “Why do we read the haftorah anyway?” we sometimes think. Krias HaTorah of the parsha makes sense—we read a portion of the Chumash each week so that over the course of the year we have completed the entire Torah. But what is the goal of reading the haftorah? We know that it is not so we can finish Navi on some kind of schedule. What then is the purpose of the haftorah?