What is the point of the discussion between Hale and the Proctors about whether or not they believe in witches ?

There are two ideas at play here. John doesn't believe in witches even though the Old Testament alludes to witches. John hence opens himself up to accusations. I think it also opens the fact that Hale is not blind to other ideas. Hale listens to Proctor and at least concedes that people try to manipulate things for their own gain.