Does punishment create a better society?

With the release of Lockerbie bomber A.M.al Megrahi a miscarriage of justice, perpetrated by the so-called international community, has been reversed. So at least a lot of people think, except people in the US it seems.

What would society “gain” from keeping this individual in prison? After all the whole case against him was very weak and circumstantial.

And why is it that churchgoers always have such strong urges for punishment?

Is punishment nothing but revenge, perhaps?