that is not the main purpose at all the main purpose is punishment, see: nonviolent drug offenders in prison one might argue a higher level purpose is to keep

that is not the main purpose at all the main purpose is punishment, see: nonviolent drug offenders in prison one might argue a higher level purpose is to keep "the right people" in prison, where they lose their freedom, sometimes including the freedom to run against, campaign against, and even vote out the politicians keeping them there