Do you think Jail/Prison is for punishment or rehabilitation?

I thought of this question after hearing about Polanski being arrested. Let’s say he has not committed any like crime in the last 20 years, that he is a good citizen now, and that was one mistake in his past, and realizes the error in his ways. Must he be punished now? I was talking to someone who lived in Europe the last 10 years, and he believes America’s thoughts on justice and punishment are all screwed up. Certainly if someone is a threat to society, routinely commiting crimes, they need to be removed, but let’s say it was one act a long time ago and now they are productive, contributing citizens?

I was curious what the collective thinks?