There are different ideas about the purpose of imprisonment but the ones I’m aware of are exclusion, deterrence, punishment and rehabilitation. The second two are predicated on prison being worse than freedom, which obviously doesn’t hold for people in a lot of situations. In fact I’ve heard of people comitting minor crimes so they can be let into prison.

Unless prisons can get really good at rehabilitation then the only way they can be effective is if life outside of prison is healthy and prosperous.