Why are prisons necessary in human society?

In the context of the novel, the whole prison house acts as a paradox. On the surface it would seem that these Puritans need prisoners (those that have fallen from God's grace) to validate their own "pious" existence. Hawthorne commences the novel by stating, “The founders of a new colony, whatever utopia of human virtue and happiness they might originally project have invariably recognized it among their earliest practical necessities to allot a portion of the virgin soil as a cemetery, and another portion as the site of a prison”. By pointing out the irony in determining the need for a prison and a cemetery during the commencement of a utopian society, Hawthorne is making a bold statement in regard to the prospect of Utopia, the fact that it does not and cannot exist in the natural world.