It's a serious question, why do we need it? Is everybody who commited a crime such a threat for a certain time, we can not let them live with the normal people. After reading 
(source: http://transgression.com.au/what-happens-when-a-prison-goes-in-to-lockdown/)
Well a short research on wiki revealed: Prison as the main form of punishment is an invention of modern days. A very interesting note is how the German Wikipedia tells a total different story compared to the English one! In the German version it says that our idea of prison as a punishment is a rather modern idea emerging somewhere in the 17th century. A time where slavery boomed as well.
The English wiki is just listing all the 'great' prisons that existed in early history and only mentions that some forms of brutal punishment were replaced by prison in the 17th century. It reads as if prison was a great humanitarian invention...
So, do we need prison? My personal answer to this: We do not. A 10 year prison sentence rarely improves the attitude of a person. I mean getting your body tatoowed all over with Aryan Brotherhood symbols is not really something that will improve your stance in society. I guess the same is true for black people getting in contact with a load of gang members in their new home.
The only real reason I hear for prisons is: You cant let those criminals run free among normal people. Here the first problem is generalisations. The word "criminals" is used as if all of them are sociopaths, while those are a clear minority. And secondly I still don't understand how even 30 years of hard labor in a hostile environment would make a murderer less likely to kill again.
My solution proposal would be a ton of psychologically educated social workers that have a very tight grip over the convicted. They could still punish them, even tho punishment is an arguable instrument for resocialization. But I think small punishments like loss of cellphone for a few hours, fees and even isolation can be ok. Isolation for a very brief time can maybe be good. 20 minutes in Isolation can feel like hours. Days in isolation, like it is often done to prison inmates, make you bat-shit crazy! I think that's just a fact.
(source: https://www.hackread.com/hacking-house-arrest-ankle-bracelet/)
This is just a raw idea of mine that I carried a long time. It's also cool that you get more demand for psycholgist, because people love to study psychology. There need to be some changes made in general to the studying of psychology, but that's a topic for another day.
Thanks 
