The only useful thing about it IMO is preventing truly dangerous/sick people from interacting with the rest of society. Most prisoners are not that
I don't think it's a stretch to suggest the US has gone long on the "retribution" angle in their approach to criminal justice.
[1] https://en.wikipedia.org/wiki/Prison#Theories_of_punishment_...
Rehabilitation: should be obvious, nordic countries are well known for prioritizing this.
Deterrence: again obvious, as noted elsewhere imprisonment has been found to be a poor deterrent in studies but the concept remains as a political argument in favor of harsher sentencing.
Incapacitation: keeping the public safe by preventing dangerous individuals from having the ability to endanger others.
and Retribution, which we just discussed above.
Only on paper (or more accurately as a self delusion). E.g. Iceland still uses excessive solitary confinement, even though it has shown to have negative rehabilitating effects and is under every definition a form of torture. The fact that the Icelandic prison system still engages in torture suggests that they are still in the retribution camp of punishment.
I don’t know about the other Nordic countries but I wouldn’t be surprised to hear similar stories from there.