Why is it that prisons are necessary in society according to this book?The Scarlet Letter by Nathaniel Hawthorne

Much of Nathaniel Hawthorne's writing is both symbolic and allegorical; and, The Scarlet Letter is no exception.  In the first chapter of his novel, Hawthorne chooses the setting of the prison and its iron door to represent the terrible restrictiveness of Puritanism that forbade sin, a natural foible of human nature.  Since sin is part of the weakness of mankind, and it was forbidden in Puritanism, it was necessary to have the prison.  Here lies the irony in Hawthorne's line that whatever Utopia is built, people must yet build a prison and a cemetery. (There is no real Utopia.)

It is especially ironic that a religious sect such as that of the Puritans, who sought the freedom to practice their religion in coming to America, should then, themselves, build a prison before all other buildings.  And, this rust and decay and ugliness of the prison forshadow the gloom of  the novel.  For, as in the works of Charles Dickens, the society in which the characters live, is virtually a prison.  Hester is labeled as an adulteress and is ostracized; Dimmesdale is tortured by his guilt about his secret sin, and Chillingworth is destroyed by his malicious acts of revenge against the minister--all are imprisoned in their own way in the restrictive Puritan society.