I think that there will be different answers here. In my mind, the film stresses the need for spiritual "redemption." All prisoners or individuals who have done terrible things must atone or find some level of acceptance for what it is they have done. Consider Red's final parole board hearing as an example of this. Throughout the film, Red had been able to give "the right line" in terms of seeking his parole and each time he had been denied. In the last hearing, he speaks honestly about regret and remorse, about atoning and reaching back into time and doing things differently. Andy understands that his own closed off and isolated nature helped to drive his wife into the arms of another, enabling her to be killed by Elmo Blanch. It is this acknowledgement that Andy receives his own redemption. It is here where the film makes its greatest statement. Prison is needed, but the realm of spiritual redemption is what must make prison effective.
It would be interesting to watch this film and then watch Tim Robbins' film, Dead Man Walking. It is evident that Robbins' work in Darabont's film impacted him or triggered a thought process on redemption and prison's role in it as he directed his own film about the death penalty. In both films, questions about the role of prisons in the lives of inmates as their quest for redemption become of vital importance.