To keep the wierdos and the rapists and the murderers and the burglars and the paedophiles out of society for the benefit of everyone.

To TRY and keep the world a peaceful and better society.

It's just like ban, keep nubs that don't know how to act right on Toribash off the site.

I think it's done to protect the community. The rest are just bonuses.

To me it seems that the primary reason should be both punishment and protecting the community.
Making them "good and honest" is only a bonus.

Whoever said "Rehabilitation", no. The government says that punishment comes before rehabilitation, except in juvenile cases.

Is punishment not a form of rehabilitation? It certainly has its place in conditioning.

It does. But it's not the purpose of it. At least says the government.

It's not bad like it was back then, they would cut your head off....

The original thought behind prison was that it was supposed to keep criminals off the streets while rehabilitating them.
Unfortunately, prison today is all about an eye for an eye. Well, of course that depends on where the prison is (country, state, etc).
Let's compare it to our infraction/ban system here. Some people ban/infract because they feel you deserve it, some because they want to give the people whose time you ruin a break and keep you off the forums, and a few because of "learn your goddamned lesson"-reasons. It's so different depending on person, just like it depends on what part of the world you live in when it comes to prisons.

I would say that the main reason is a tie between; Rehabilitating criminals, and keeping those criminals away from the public to relieve the stress of families that possibly have kids and need to worry.