[uptheroots]

I believe prisons in part serve the purpose to keep us in check. With the threat of prison, society is encouraged to police itself. As another comment in this thread mentioned, Foucault helped to develop these sort of ideas and describes the motivations and development of the prison system in his work.

[Tao3300]

It keeps us in check but not like you think. At least for crimes with direct victims, prisons keep us out of blood feuds by letting the state monopolize retribution while keeping the object of it out of reach.