PorcelinaV, mmmmm, good question. I think because rehabilitation will probably result in a more productive end in the long run than punishment.
Would it not be better for society as a whole if a criminal member of it was turned into a non-criminal member? Imo it would be, anyway! But it seems to be a bit of a pipedream, frankly, and doesn't happen often enough, surely enough. It might be that we need tougher sentencing - well, actually I think it's quite clear that we do - going hand in hand with more effective and more plentiful rehabilitation programmes. I confess I don't know how this all is atm, so for all I know there are swift, effective and plentiful rehab programmes running in every single prison in the UK, all producing model citizens at a rate of knots; it just doesn't look like it from where I am.
I also think that if punishment is the only point, or main point, of bringing someone to Justice, then we find that sentencing becomes inadequate, we increase sentences, prisons get more overcrowded and more expensive, leading to more prisoners who hold a grudge against society as a whole, leading to more crime, leading to harsher sentences, and all the while crime is increasing.
Dh, otoh, thinks that criminals generally just want to be criminals and nothing you do will change that, regardless of sentences or rehab measures.