Mary Warren is caught in between opposing factions in The Cruciblebecause she has the power to help both sides prove the truth of their argument.
The court (Danforth, Parris, etc.) believe that witchcraft is truly happening in Salem. Mary Warren pretends to see spirits and suffers from (acted) fainting spells in court. Later, when Proctor brings her to court for his own purposes Mary Warren betrays him and again joins Abigail's group of girls, pretending to see spirits.
Proctor, as Mary Warren's employer and as her moral advisor, attempts to have Mary Warren tell the truth to the court. If he can convince the court that the girls are faking their fainting spells and only pretending to see spirits, he can prove that the accusations against his wife are also false. This is his purpose in bringing Mary Warren back to the Salem court.