Why does Mary Warren spend the day in Salem in The Crucible?

Mary Warren goes to Salem because she is an official in the witch trials.

Proctor forbids Mary Warren from going into Salem. When he learns that she has gone anyway, he is angry.

Elizabeth: Mary Warren’s there today.

Proctor: Why’d you let her? You heard me forbid her go to Salem any more!

Elizabeth: I couldn’t stop her.

Proctor, holding back a full condemnation of her: It is a fault, it is a fault, Elizabeth - you’re the mistress here, not Mary Warren. (Act 2)

Elizabeth tells him she is afraid of Mary, and he says she would be afraid of a mouse.  She explains to her husband that Mary Warren told her that she had to go into Salem because she was “an official of the court.”  Elizabeth is afraid of the court, but her husband tells her it is nonsense and "black mischief."

The couple argues about Abigail, who had an affair with John.  Abigail is the one who is responsible for starting the witch trials with her antics and her lies, so it is more serious than they realize.  It will make trouble for them later.

When Marry Warren finally comes home, John Proctor accosts her.  He grabs her cloak and threatens to whip her.  She doesn’t resist him, but she does object, saying she is sick.

Mary Warren: I am sick, I am sick, Mr. Proctor. Pray, pray, hurt me not. Her strangeness throws him off, and her evident pallor and weakness. He frees her. My insides are all shuddery; I am in the proceedings all day, sir. (Act 2) 

John tells her that her place is in his home, because she is his servant.  She is shirking her duties by sitting on the court.  Mary’s response is to give Elizabeth a doll, saying it is a gift she made during the proceedings.  This doll will later become important, because Elizabeth is accused of using it like a voodoo doll when a pin is found on it.