The main benefit of prison is obvious. For those who are too dangerous to be anywhere else, we have a vaguely humane place to put them to keep the rest of society safe. And if prison was only used in this way, perhaps this would be a less ranty pissed-off article.
Unfortunately, as I'll discuss later on, prison is used way too much on wee lil' trivial crimes. But before we go there, let's explore why prison doesn't even really work for the murders/rapists/baddies.
So if you get caught with some weed and go to prison, after you emerge and integrate back into society, you are now more likely to commit crime than if you had never been to prison. Problematic right? Why are we spending £34,000 per year per inmate to effectively teach people to commit crime, and encourage their recidivism? It's for this reason that prison has been called a "University of Crime."
Part of the problem here - of why people reoffend more after being detained at Her Majesty's Leisure - is that we don't do a very good job of rehabilitating people.
Ideally, we'd be like Norway. Where offenders go in, learn some essential skills (E.G. learning a trade, attaining higher levels of maths and English, therapy and behaviour counselling) and then pop out right back into society. They would get a job, and then pay tax, and become a functional egg.
Availability of help and courses to grow skills is a pretty key component of rehabilitation. Unfortunately in England and Wales, our prison service funding has been cut pretty ruthlessly over the last decade. We don't have enough staff to man prisons, much less teach a cooking course. We're slowed down by bureaucracy to such an extent that offenders have often finished their sentence by the time a spot becomes free.
With nothing else to do other than hang out with other criminals, and no healthy outlet for the negative feelings you'd surely accumulate while incarcerated, and no support to get better and address the issues that lead to offending - is it surprising that people reoffend more after prison?
Regardless of weather the offense was that persons' fault or not, the most economic way to deal with a crime is to treat the problem that caused the person to offend.
We send people in who have problems - and they definitely do, whether it's mental illness, or lack of anger control, impulsivity, whatever it is, they have a fucking problem - and then we do nothing to try and solve this problem, and release them. Why would we expect anything to change when we do not change they're circumstances? If anything sending them to prison (decreasing their chances of getting a job, straining their relationships with friends and family) means their lives are now much, much harder than before they went in.
Einstein famously said that madness was repeating the same exercise without changing variables, and expecting change. This is exactly what we're doing, with huge expense to the tax payer.
Secondly, there seems to be this weird assumption in society that prison acts as some kind of deterrent, but this just isn't the case. The Law Commission itself acknowledged in it's recent report that (Law Commission, 2020)
The Law Commission itself acknowledged in its recent consultation report that "there is no empirical evidence that higher sentences are capable of deterring offenders.” (Law Commission, 2020).
So we can see that prison in general, doesn't really work. Well, doesn't work for most. Prison does certainly work for the people who we can do nothing else with but incapacitate. However, prison is largely used not for these people. It gets used for shoplifters and burglars - property crime that poses a significantly lower risk to society than that of murders or kidnappers or rapists. A.K.A The people that prison was meant to be used for.
"[A]n offense has to be so serious that neither a fine nor community sentence can be justified"
So for the majority who are imprisoned, it doesn't work. An area of particular interest is women in prison. I was very surprised (although on reflection, I probably shouldn't have been) to learn that the mental health of women in significantly worse than that of men.
Despite making up only 5% of the prison population in England and Wales, women make up half of the total self harm incidents in prison (Prison Reform Trust, 2005). A much higher percentage of the woman have experienced domestic abuse and sexual violence (Corston, 2007) and as a result are more prone to mental illnesses like depression and anxiety.
However, while women suffer more in prisons - self harming and killing themselves at a much higher rate than men - women are more likely to go to prison for less serious crimes, when compared against their male counterparts. For example, if a woman shoplifts and a man shoplifts, the woman is more likely to go to prison, even though the crime is the same.
We can see this weirdness reflected in statistics of the offenses that men and women go to jail for. Half of all women in prison right now are there because of property crime, so that's theft, shoplifting - nothing violent (Carlen, 2013).
So on reflection, we can see that prison is used far more than neccesary - at huge public expense. And once those people are on the inside, they instantly become more likely to re-offend. The idea of having your liberty taken away, does not act as a deterant. And if you happen to be female, the chances of you self harming and suffering mentally are extremely high.
Begging the question - why are we doing this?
If you're doing some laundry or cooking and want a pretty dry soundtrack - listen to this ace podcast. It's funny and summarizes the main points really well.

Refrences:
Carlen, P. ed. (2013) Women and Punishment. Oxfordshire, Taylor and Francis Group
Corston, B.J. (2007) The Corston Report: A Report of a Review of Women with Particular Vulnerabilities in the Criminal Justice System. London, Home Office
Law Commission (2020) Hate Crime Laws: A Consultation Paper. London, Law Commission
Prison Reform Trust (2005) Prison Fact File May 2005: Bromley Briefings. London, Prison Reform Trust