Some want prison to be a hard punishing experience and are fine with low quality of life and forced labor, because they believe that this sort of punishment will make criminals regret doing crime, and discourage future people from doing crime. This is the same logic that leads to supporting the death penalty (deterrence).
Others want more of a Nordic model, where prisoners are given essentially a basic but relatively-well-appointed studio apartment, good food, activities, etc - but also with a lot of mandatory therapy and "social rehab" classes and stuff like that, in order to reform prisoners back to model citizens.
Really what it comes down to is what we believe is the source of crime. Are criminals regular people who made a bad choice (and thus the source of the decision can be found and rectified)? Or are they a different kind of person more likely to do crimes (and thus must be locked away from society without bothering to try and "rehab" them)?
Like anything it is an incredibly complex and nuanced problem. But personally I think we should all agree that if the State is going to maintain the power to lock people up, we shouldn't allow doing so to become a profit center.
