> What does society gain from a prison like that?
No, you have to ask, what does a prison like that gain from society?
Once you understand that people are merely a resource for government, law enforcement and business, things make more sense.
Think about that as you drive by a speed trap, that the police and city undoubtedly portray as a safety operation, but where the cops hide themselves because they're not trying to discourage speeding, they're trying to collect taxes.
Think about that when you see various police organizations stating their opposition to legalizing marijuana. Is their objection that it may promote other, still illegal drugs, or that it would reduce LEO job opportunities? Their only proper stance on the issue, as LEO organizations, is to enforce the laws that society tells them to.
Think about that as an apparently overzealous prosecutor bullies a plea bargain out of someone with the threat of life in prison. She's not overzealous, she doesn't care about society; the victim/defendant is merely a line on her resume. When a prosecutor's job performance depends on convictions (rather than truth), then she's going to maximize convictions.
Think about that as you, just a normal citizen who is no threat to anyone, are monitored, threatened and controlled by unconstitutional surveillance and police tools that are supposedly intended for, you know, actual terrorists threats.