I've said this before, but the reason for Prison is based on the type of justice model that a country uses. In the USA, which has the highest incarceration rate in the world, justice is largely punitive. If this model were any good, then they would not have the highest incarceration rate in the world. Once you are in the prison system over there it's incredibly difficult to leave it.
In the UK, we have a restorative justice model(ish) although the longer we have people like Priti Patel in charge, the more punitive it becomes. Hence the Daily Fail and Tories calling for "tougher prison sentences" - they know it sits well with voters.
A good example of restorative justice is Portugal who decriminalised drugs in favour of treating drug addicts (drug dealers still go to prison).
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They also found a huge reduction in social costs and HIV diagnoses, thus saving money on healthcare and courts.
Personally I favour a restorative model where only the truly dangerous remain incarcerated and those that can be turned round to being useful members of society are helped. Whilst grieving relatives want dangerous drivers to go to prison for life, I'd rather that they were tagged, working and part of their income used for good (e.g. a fund to help grieving families). I don't see how a roof over your head and 3 meals a day in a low security prison is going to achieve much except cost the taxpayer money which could be spent on the NHS or the Justice system, or improved services to reduce reoffending.