In The Crucible, why is Mary Warren considered an official of the court?

Readers find out that Mary Warren has been appointed to "an official of the court" in a conversation between John Proctor and his wife, Elizabeth. Elizabeth says,

"I forbid her go, and she raises up her chin like the daughter of a prince and lays to me, 'I must go to Salem, Goody Proctor; I am an official of the court!'"

This conversation escalates the plot significantly; it's where readers learn that a "proper court," with magistrates from Boston, is coming to Salem to investigate the allegations of witchcraft. Mary Warren herself illustrates the connection to Boston in an earlier conversation with Abigail, citing that there had been hangings in Boston of accused witches. She says,

"Abby, we've got to tell. Witchery's a hangin' error, a hangin' like they done in Boston two year ago!"

Mary Warren is emboldened by the title of court official. She goes from being an insignificant maid for the Proctors to being what she perceives as being an instrumental, necessary part of the court process. She basks in the power, telling the Proctors she is now allowed to come and go as she pleases and they can no longer give her orders. 

It is likely, however, that the main reason the court appointed Mary Warren as an "official" was to gain information about her activities and those of her peers. Mary Warren, though, considers it a role that places her above even John and Elizabeth Proctor. She acts as a "mole" for the court, reporting back to them conversations and actions she has heard and seen. 

Mary Warren's shift from maid to official signifies a shift in the plot, as it gives the court a direct eye into the Proctor household; this eye ultimately contributes to John Proctor's hanging.