The Formicarius is not primarily about witches. It is a long moral work in five books by the Dominican theologian Johannes Nider, cast as a conversation between a theologian and an idle student, and organised around the ant colony as an image of ordered Christian life. Four of its books are about visions, revelations, false piety and the state of the church.
The fifth book is why it matters. In it Nider gathers accounts of people who, he says, have renounced the faith, taken up with demons, killed and eaten infants, and gathered to do so together. This is the earliest sustained description in Latin of witches as a sect: not individual sorcerers working charms, which every earlier century knew about and mostly punished lightly, but an organised counter-church with initiation, obligations and meetings.
That change of category is the whole of its importance. A sorcerer harms a neighbour and can be tried for the harm. A member of a conspiracy against Christendom has accomplices, and a court that believes in the conspiracy has a reason to keep asking who else was there. Almost every motif that would later appear in confessions across Europe can be found in outline here, two generations before the Malleus Maleficarum and three centuries before the worst of the trials. Nider was writing in the 1430s, around the Council of Basel, at a moment when the church was rethinking heresy, authority and reform, and the shape he gave the crime long outlived the arguments he built it for.