One thing I haven't read anywhere in reviews of rap is how it often draws on the 'shadow side' of our natures. The term 'shadow side' comes from Carl Jung. He described the shadow side as the part of who we are that we don't acknowledge. Part of the the shadow side is made up of things that we push away because they are too difficult to deal with (our own anger, for example). Part of it is things that we haven't developed (like the neglected creative, imaginative side of someone who has worked at Goldman Sachs all their life). Someone who has a very narrow personality - or a 'persona' that they put on - probably has a large shadow side. For example, someone who has a macho personality all the time is probably pushing many things down inside themselves and hiding much of their full nature.
So much of rap is about bringing the shadow side to the front. Eminem for example, had a childhood where he was often a victim and was often beaten up. As he raps about, he often just sat there and let it happen to him. Probably he buried his anger. Then he discovered rap, which was a form where he could express anger - in a way that is socially impressive. The training in the rap battles in Detroit probably gave Eminem a chance to get in touch with his anger, which had been pushed to his shadow side, and bring it into his conscious, expressive side. Which is why so much of his rap is angry. And it connects with anyone who had suppressed anger. With Eminem the example is almost perfect, because he also created a new persona, Slim Shady, which literally stood for his shadow side. Creating Slim Shady was the turning point for him, that really unleashed his creative powers (made awesome by his great lyrical abilities). Joyner Lucas, another awesome rapper and a friend of Eminem's, also struggled a lot in life and raps about that. And so many other great rappers too.
In the past, court jesters were the ones who were given official permission to represent the shadow side of things, to the King or Queen. Everything that members of the court would deny or couldn't talk about because of social pressure, the jester could bring to life via jokes. The jester is a necessary role because we all are pulled in two directions: (a) we have to be 'civilized', kind, and fit in with the social groups we care about, which inevitably involves pushing down some frustration, infantile or wildly creative desires, and (b) we have to live with all the stuff we push down inside ourselves. As Freud pointed out, in any civilized context, there will be the 'discontent' people or the discontent parts of ourselves. Via a court jester, comedian, rap music, therapy, or some other means, we need a way to access that hidden, shadow side in a way that is somewhat controlled and socially sanctioned.
Reviewers and critics miss the point when they think that rap music is about promoting the 'shadow side' (for instance, Eminem's recent song 'Darkness' where he raps from the perspective of the Las Vegas shooter). Rather it is about trying to cope with and process this side of life. It's actually about taking the fear and anger out by bringing them in to music, making these things part of our collective consciousness, rather than pushing them away and pretending no shadow side of life exists.