Reverend Parris is a very important man in Salem. As a clergyman, he has considerable personal, spiritual, and political authority in this Puritan town. He's generally respected, if not exactly loved. When he discovers that his niece, Abigail Williams, has been engaging in strange rituals in the forest at night with some other girls, he's horrified at what this might mean. On the face of it, it seems that the girls have been indulging in paganism, possibly even witchcraft. At the very least, they've been behaving in ways that are universally considered unacceptable among Christians.
Parris is worried that if news of what happened in the forest gets out his reputation will be ruined. So when his daughter Betty gets sick, he cynically seizes the opportunity to turn Abigail's weird cavortings to his advantage. The doctor cannot cure Betty of her illness and suggests that there are dark, demonic forces are at work. Parris is convinced that witchcraft is somehow responsible, and with his niece's able assistance he exploits the community's fear of witches to whip up a firestorm of mass hysteria. Not only does the witch-craze divert attention from the strange goings-on in the forest that night, it also allows Parris to consolidate his authority over the people of Salem.