Sir Thomas decides to stage a ball to introduce Fanny into society. Up until this time, Fanny has occupied an undefined role at Mansfield Park. She is of comparable age to her cousins but not really a peer of theirs, as she comes from a lower social sphere. Therefore, Fanny does not attend the social events and balls hosted by other wealthy families in their neighborhood. Austen writes,
Fanny had no share in the festivities of the season; but she enjoyed being avowedly useful as her aunt’s companion when they called away the rest of the family; and, as Miss Lee had left Mansfield, she naturally became everything to Lady Bertram during the night of a ball or a party.
Attending a ball is one sign that a young woman is eligible to marry within the social circle of those attending the event. Fanny is not “out” in this sense. This becomes clear when Miss Crawford questions Edmund about whether Fanny attends balls:
“No,” replied Edmund; “I do not think she has ever been to a ball. My mother seldom goes into company herself, and dines nowhere but with Mrs. Grant, and Fanny stays at home with her.”
“Oh! then the point is clear. Miss Price is not out.”
Fanny not being “out” would mitigate her prospects of marrying Henry Crawford or another young man of comparable social status. Nevertheless, Fanny has enjoyed an education that is comparable to the education her female cousins have had. Moreover, reflecting her superior intelligence and knowledge, she can far outshine Maria and Julia Bertram in her understanding of most subjects, including current events. She is articulate and well read, with a keen understanding and sensitivity to other people in their social circle. She has also evolved into a lovely looking young woman.
Therefore, Sir Thomas understands that by staging a ball and introducing his niece into society, she will achieve a social acceptance that she has not held before. Sir Thomas also knows that Henry Crawford is interested in Fanny, and he is keeping their union as an option in his mind. The ball and Sir Thomas’s role as host will give Fanny the cachet to rise above her birth station and therefore make her eligible to marry a man of higher social consequence, such as Henry Crawford.