Why do we put people in jail?

Okay I understand we put people in jail or prison for punishment on their bad actions. But I feel like this should be temporary. Like there are people who have raped others, and all we do is throw them in jail, but I feel like we teach and help these people. Nobody is born to do this terrible acts, but if someone does, I feel like jail does NOTHING. It removes you from society (in some cases it's NEEDED) but in most cases it don't teach them anything, they just adapt and can't wait to get out. But if we can teach and explain. That can actually make a positive difference.

People think when we scare people with jail or prison is going to stop people from doing bad things but it doesn't. We need a little jail, teaching and other serious help for these people. And no, those little classes aren't good enough because it's a choice to go or not. SOME PEOPLE NEED IT. I just don't understand why we see someone kill someone, and we just through them in jail for 15 plus years and think that's supposed to help the situation somehow