Prison is like an internment camp, but with a process of trial. Trials are when you buy the most expensive lawyer you can to show how rich you are, and if you're rich enough; Then you don't have to go there.
Prison is also a place where a criminal is put. So that they can find other criminals and make business contacts. These business contacts will then become very useful since nobody wants to hire a convict. In prison people can also take the time to educate themselves in how to become better criminals, and figure out what they did wrong by sharing tips and ideas with other prisoners.
Prisons are also where gangs recruit most of their members, and is one of the many institutes devoted to the sheer stupidity of the human species.