Prison serves three functions, one in two ways. Which is the "prime" purpose is a political construct of a given time.

THey are: -

1. Denoting societal disapproval (punishment)
2. Prevention of re-offending - because teh offender is not free to re-offend (incarceration)
3. The discouragement of future offences
(a) By the offender through treatment and rehabilitation
(b) By the offender or others through fear of punishment.