Revealing mistake: Right before the witches burst through the roof of Max's house, after they retrieve the book, you can see the charge used to set off the explosion in the window just below where they exit.
Revealing mistake: As Mary flies toward the kids in the cemetery, the wires holding her are visible.
Answer: They were set up, beforehand, just in case. Winnifred knew about the spell, but she didn't memorize the incantation. So she needed the book to see how to cast it.
Phixius ★
If you pause the movie, you will see that the book opened up to the Black Flame Candle spell.