Sentences have 4 functions (+ the profit function in case of the US).

• Prevent that the same person does a crime again (that is the reason why even after prison sentence or instead of you can out people in psychiatry)
  and prevent that other persons do the same crime by shocking them
• protection of society, prevent someone gets hurt
• rehabilitation
• retaliation

the first point and second point works - as long as the person is in the prison - for the person, but after? Also, as the US shows, deterrence is not very effective
Reha is needs effort, and that is often lacking
Retaliation - I think that is a big part in the US