Putnam tells Parris that he needs to make the announcement himself that he has discovered witchcraft in his household. He tells Parris that if he takes control of the situation now, and tells the town that he is going to take on the task of ferreting out the source of the witchcraft and find out who the witches are, the town will praise him for it. Parris is hesitant for fear the townspeople will look down on him since his daughter is one of the "afflicted". But at Putnam's urging, Parris realizes that he can use this to his advantage if he makes it to seem that his daughter is a victim of witchcraft, not a practitioner of witchcraft. This helps to feed the fire of frenzy and fear that is burning through Salem. Putnam sees this as a chance to control Parris and to get his neighbor's land.
I'm not convinced that Putnam has already realized how he can manipulate this situation to his advantage. He says plenty to suggest that he really does believe that witchcraft is to blame for the loss of seven out of his eight seemingly healthy (at first) children. When Parris begs Putnam not to leap to witchcraft as the cause of Betty Parris's and Ruth Putnam's illnesses, Putnam says, "I have taken your part in all contention here, and I would continue; but I cannot if you hold back in this. There are hurtful, vengeful spirits layin' hands on these children." In other words, he's been Parris's ally in all previous disputes in Salem, but he can no longer unless Parris takes this bull by the horns.
When Mrs. Putnam confesses that she sent her Ruth to Tituba to conjure the spirits of her dead babies, Putnam says to Parris, "Let your enemies make of it what they will, you cannot blink it more." He means that Parris is going to have to address this; it must be out in the open. He advises Parris to try to get in front of the accusations, though. Parris is terrified that he will lose his position, but Putnam advises him, "Wait for no one to charge you -- declare it yourself. You have discovered witchcraft--." If Parris takes the credit for recognizing the signs of witchcraft in the village, then he will become more powerful, not less. Putnam tells him, convincingly, "Strike out against the Devil, and the village will bless you for it!"