The overarching goal of the city is to educate those with the right natures, so that they can turn their minds sharply toward the Form of the Good. Once they have done this, they cannot remain contemplating the Form of the Good forever. They must return periodically into the cave and rule there. They need periodically to turn away from the Forms to return to the shadows to help other prisoners.
It is important to realize, when reading the allegory of the cave and of the line, that Plato means to depict not only four ways of thinking, but four ways of life. To use an example, imagine that a person in each of these stages were asked to say what courage is. The understanding of courage would differ widely from stage to stage.
Working with a possible interpretation of the imagination stage, an individual’s notion of courage in this stage would appeal to images from culture. Such an individual might try to explain courage by saying something like, “Luke Skywalker seems really courageous, so that’s courage.” An individual possessed of beliefs would also appeal to a particular example, but the example picked would be drawn from real life. There might be mention of the Marines or New York City firemen.
Someone at the stage of thought, in contrast, will try to give a definition of courage. Perhaps they will give the definition offered by Socrates in Book IV: courage as the knowledge of what is to be feared and what is not to be feared. What separates the person speaking from thought from the person possessed of understanding is that the person speaking from thought cannot inform his views with knowledge of the Form of the Good. They are working with unproven hypotheses rather than the true first principle. Even if their definition is correct, it is left open to attack and objection because their grasp of the relevant concepts stops at a certain point. Speaking from understanding, someone giving a definition comprehends all the terms in the definition and can defend each one of them based on the first principle, the Form of the Good.
Because the Form of the Good illuminates all understanding once it is grasped, knowledge is holistic. You need to understand everything to understand anything, and once you understanding anything you can proceed to an understanding of everything. All the forms are connected, and are comprehended together in the following way: you work your way up to the Form of the Good through thought until you grasp the Form of Good. Then, everything is illuminated.
Since the stages in the cave are stages of life, it seems fair to say that Plato thought that we must all proceed through the lower stages in order to reach the higher stages. Everyone begins at the cognitive level of imagination. We each begin our lives deep within the cave, with our head and legs bound, and education is the struggle to move as far out of the cave as possible. Not everyone can make it all the way out, which is why some people are producers, some warriors, and some philosopher-kings.