Parris calls in Reverend Hale in order to calm the fears of the townspeople in Salem (and to protect himself -- he fears being ousted by an enemy faction). Rumors are already circulating quickly about his daughter Betty. Some people are saying that she's possessed; others say that they saw her fly over a neighbor's barn. Moreover, rumors are circulating about the reputation of his niece, Abigail, as well. Parris is terrified that people will believe that witchcraft has infected his home, and, as Mrs. Putnam says, everyone will think that "It is surely a stroke of hell upon [Parris]." He will lose all credibility if this rumor persists. Therefore, Parris is desperate to get ahead of the hysteria and declare that there is no witchcraft in his house (or Salem), so he calls in a well-known expert, Mr. Hale, to confirm his view. He believes, after all, that Hale will find no evidence of witchcraft, as he tells Putnam that calling Hale is "A precaution only."
Parris is nervous during the opening scene of The Crucible because his daughter is paralyzed. As the town's resident minister during the late 1600s, there is great fear that the daughter may be possessed. This would be a great problem for his reputation. In fact, his career could be ruined. Although it would be great to have a doctor just fix an illness, Reverend Hale of Beverly is a known expert on these issues. Calling him in can confirm that she isn't possessed. If by chance she is, then this Reverend Hale should be able to exorcize the demon possessing her because he already apparently healed another woman of this type of ailment.
Parris calls in Hale because it was said that Hale was an expert in witchcraft.