What is the cause and effect of "The Allegory of the Cave?"

In this allegory, people are in a cave, chained to a wall in such a way that they can only face one way and essentially have no knowledge that there is anything behind them. But behind them are puppet performers and behind the performers there is a fire. Further behind this is a passage that leads outside the cave to the sunlight. The people in the cave only see the shadows on the wall the face. The shadows are created by the performers. So, the chained people only see a semblance of the truth. The absolute Truth lies outside the cave in the sunlight. So, the people in the cave are thrice removed from absolute Truth: Sun/Truth - Performers/Fire - Shadows. 

This allegory shows that there is more to truth or "reality" than what we see and experience. In Plato's Idealist philosophy, the allegory is meant to show how his abstract Forms (Ideal ideas of things in their perfect form of Truth) are three times removed from our normal experience. The allegory also suggests that philosophers or the philosopher-king rulers he talks about in The Republic, should (after they become enlightened in seeing the Truth/Sun) go back into the cave (back to the city/society) in order to rule and educate the masses. 

This allegory is a classic structural way of presenting the notion of "thinking outside the box." In this case, the box is a cave, but Plato has other applications of the metaphor, as discussed above. 

What causes people to experience things so far from Truth? This is a broad topic, so there is too much to go into here. But consider things like culture, ideology, our fallible senses, and tradition as things which structure our thinking and behavior. Plato was "caused" or motivated to write the allegory to illustrate how people should aspire to greater truths that they should go above and beyond traditional ways of thinking. The hope is that this will create many effects: a more educated public and more particularly, a more educated body of leaders. 

In the history of Western Philosophy, this allegory has had a great effect on subsequent thinkers. Philosophers have debated different interpretations of the allegory but all consider it to be one of the most important allegories of seeking truth and wisdom.