For Reverend Parris, the most important thing in the world is his reputation. And as his actions reveal throughout the play, he's willing to sacrifice the lives of innocent people to maintain that reputation at all costs. When he discovers what his niece Abigail's been up to in the forest, he immediately starts to think of the damage that could be done to his good name. The niece of a prominent clergyman involved in what looks suspiciously like witchcraft? The scandal would be too much to bear.
When Abigail starts making false accusations of witchcraft, Parris gladly goes along for the ride. Not only will the witch-craze divert attention from Abigail's weird pagan cavortings in the forest but it will also give Parris a golden opportunity to burnish his reputation among the people of the town and consolidate his political power. And that, more than anything else, is what's most important to him.