Can't it be for both? and for incapacitation.
If it had to be for one then I would say rehabilitation, although for more severe crimes incapacitation becomes more important. Both of these things protect other people, while punishment doesn't protect anyone so for me is a less important consideration.
Although I suppose you could say that currently the main 'purposes' are incapacitation and punishment. People also claim that the existence of prisons acts as a deterrent, but I'm not convinced, and I'm also not convinced how well people are rehabilitated. I just think that rehab is more important.
I see little justification for "punishment" aside from vengeance... rehabilitation and the protection of the public, I should think.
If they don't understand that actions have consequences they won't learn discipline. However, if they just get punished all the time they'll become angry and bitter. There has to be a balance of both.
I really don't like the idea of punishment for the sake of punishment, it just seems so pointless and only satisfies some peoples desire to see others suffer.
Punishment for the sake of deterrent has merit, but we need to be sure that we are punishing in the right way then. I think that prison should be a lot about rehabilitation so we can get people out of prisons safely. This could even save money long term.
Both to an extent. But it IS there as a punishment as well. It is right that those who do wrong (in this case break the law) receive punishment for it. It's not so much vengeance as much as it is a deserved result of their actions.