Poor Guster, the Snagsbys' maid, is given to seizures. Why does Guster fit in so well with the Snagsbys?

She is a hard worker, she is not attractive to the apprentices, she gives Mrs. Snagsby someone to criticize, and Mr. Snagsby feels he is being charitable by keeping her. Likewise, Guster has a deep appreciation for her surroundings and does not want to go back to the workhouse from whence she came.