What Good does Prison do?

Prison locks away people who have "wronged" society for years. Even being jailed for a few months can be very harmful to the life of anyone still in school.

Ten years and their life is ruined. (not just school kids) Their loved ones might have moved on, they become angry, and gain nothing. Being locked in a cement box will not make you a better person. All it does is satisfy the selfish desire of the "victim" for revenge.

Well, that's how I see it.

I'm sure people here disagree.
Please, enlighten me as to why. (Not sarcasm)