Civil societies have a duty to protect themselves from the actions of members of the society who fail to abide by the rules of the society. In modern society, these rules are established by representative bodies (Congress) and enforced by the executive. Infractions of the rules are called crimes. A police force is often the agency that identifies the commission of a crime, a prosecutor specifies the charges and makes the case before a tribunal or court, and a corrections department carries out the actions prescribed by the court. Often, the actions consist of confinement to prisons for some period of time, financial penalties or probation.

Why do societies do this? There are several purposes, some practical, some with unintended consequences, and some to assuage anger and pique. Lets look at some of these effects. The first goal for a society is to reduce crime. Numerically, just locking up people who commit crimes is likely to decrease the instance of crime as long as that person is in prison, because it has been shown that people who have committed a crime are more likely to commit another than the general population. To the extent that you take a thief off the street for a few years, that thief will have no opportunity to steal for that period.

It has also been shown that when people who have committed crimes reach middle age, their likleyhood of committing additional crimes is decreased. So, the portion of a person’s 25 to 30 year crime career that is consumed by prison is that much less crime that they get to commit.

Punishment for criminal behaviour is a goal often stated. The premise that “bad things” will happen to you if you commit crimes, expects punishment to be a deterrent to the commission of crimes. Societies often also want retribution for crimes that have been committed; society should “get even” for the crimes. Biblical punishment of an eye for an eye (Exodus 21:23-27) is a basis for mirror punishments. One can argue the case that punishment of this type will be a deterrent to others against commission of the same crime. It also quenches a thirst for vengence against criminals, particularly those who have committed grevious crimes.
Each of these are crude weapons against crime. The first, warehousing criminals, is like bailing the ocean and is an expensive approach to the problem, both in the direct costs (about the same as the cost of sending a person to college), and the indirect cost of losing the individual as a productive member of society. The second, punishment, might provide some deterrent, but is unlikely to do much for crimes committed on impulse, where consideration of punishment does not considered.