Mary Warren informs Elizabeth that she is an official of the court and must go into town that day. This is important because it shows the level of power that has been given to the girls, allowing them to be disrespectful to their authority figures, something that they previously would have been beaten for.
Elizabeth sees how out of hand this is getting, specifically because Abigail is involved. Elizabeth is one of the few characters in the story that is able to see through Abby's lies. Elizabeth wants John to talk to Cheever about Abby's claim that it has nothing to do with witchcraft.
Since the first accusations were made in Reverend Parris's house, Mary has been a part of most of the proceedings in the courtroom. So when Johh Proctor demands to know where she has been all day her response is that she is "an official of the court now", meaning, she has been given a title of "official" at the court, and is duty-bound to be there for the proceedings each day. Also, she feels it it is "weighty work" that she does; for once, Mary Warren feels important, wanted, listened to, and brave. So, she tells John that she will "be gone every day for some time." However, once Elizabeth is arrested, she stays at home "strivin' with her soul", trying to get the true courage to go against Abigail and all of the other girls.
Both answers suck #@%$