What Good does Prison do?

I entirely agree - I don't think that prison helps the criminals that matter.
For example, say that you're having an argument with somebody. That person locking you in a room full of people that wholeheartedly agree with you isn't going to help their case.

Plus it wastes space, money, etc. But really, I don't see any viable alternatives :/;; with rehabilitation too expensive and massive of an effort, and capital punishment is wrong in all but the most severe situations imo. Maybe memory-haxing instant solitary confinement could scare a small-time criminal straight, but I'm worried about fooling around with that stuff.