1 Answer

There are a number of accepted reasons for the use of imprisonment. One approach aims to deter those who would otherwise commit crimes (general deterrence) and to make it less likely that those who serve a prison sentence will commit crimes after their release (individual deterrence). A second approach focuses on issuing punishment to, or obtaining retribution from, those who have committed serious crimes. A third approach encourages the personal reform of those who are sent to prison. Finally, in some cases it is necessary to protect the public from those who commit crimes—particularly from those who do so persistently. In individual cases, all or some of these justifications may apply. The increasing importance of the notion of reform has led some prison systems to be called correctional institutions.

http://www.britannica.com/EBchecked/topic/477205/prison/272178/The-purpose-of-imprisonment