Exactly why (originally) is Rev. Hale in Salem in "The Crucible"?
Reverend Hale is specifically summoned as an expert in detecting the presence of witchcraft. He is asked to come to Salem by Reverend Parris to examine his daughter Betty, after she falls into an unknown illness.
Reverend Parris hopes that Reverend Hale will confirm that Betty's illness has nothing to do with the supernatural.
In Act I, Scene I, Reverend Parris says referring to Betty's condition:
"There be no unnatural cause here. Tell him I have sent for Reverend Hale of Beverly, and Mr. Hale will surely confirm that. Let him look to medicine and put out all thought of unnatural causes here. There be none." (Miller)
Reverend John Hale is a minister of great book learning and much respect. He is from the Boston area where there have been troubles with accusations of witchcraft in which he was able to determine the cause and help resolve the matter. He is summoned to Salem for the same reason--to determine if they do, indeed, have a "witch" problem. He is sort of a comic character appearing with all his books and lumbering down the road, but he is one of the only people in Salem to recognize the innocence of John Proctor regardless of the fact that he is too weak a character to do anything about it.