Why would Hawthorne want to include Mistress Hibbins as a minor character in this book?

Mistress Hibbins is the resident witch. We are not sure if she was a witch in real life but the village seems to think so. Mistress Hibbins is a widow who lives with her brother, Governor Bellingham, in a luxurious mansion. I suppose being a widow was half way to becoming a witch if you were a Puritan. Apparently Mistress Hibbins goes into the forest and pals around with a "Black Man" at night. She shows up now and again to remind the already paranoid Puritans that evil is just around the corner and that they should all be double paranoid if they see their friends and neighbours petting a black cat or reading a book other than the Bible. She really serves to illustrate hypocricy in Puritain culture.