I'm mostly with Dutch here- somebody said to me once that prisons are full of "The mad, the bad, and the sad" and that only one of those should be in there. The purpose of prison is to protect society, not to make society feel good about what it has done to punish the bad people.
So people should be in prison because it's in society's interest to simply remove them (that in particular is the habitual re-offenders, predatory paedophiles, etc.) and those who are considered capable of reform but need to be isolated whilst they are treated.
I do disagree with Dutch insofar as I believe that reflection and self awareness can be taught, for some people. Whether prison is the right place for that, of course, will depend on circumstances - sometimes it'll be true, sometimes it probably isn't.