Prisons serve to remove a person from society (in the civilised world).

Removing a person from society has two subtly different justifications:

1. Prevention: their offence against society is such that society must be protected from them.
2. Punishment: their offence against society is such that society wants them deprived of society.

Consider the difference between violent crimes (1) and nonviolent crimes (2).

Note that some societies expand (2) to include more punishment than just separation from society.