Social structure in the novel is used to reveal Emma's pride and arrogance, and it is the central theme in her misguided matchmaking attempts. Emma experiences a change and becomes a more humble person when she realizes that she had focused so much on social structure that she had become blind.
Emma uses her amateur matchmaking skills to find a husband for Harriet Smith. In Chapter III of Emma, it is made clear that Mr. Woodhouse is a man who spends most of his time with people from his social class. As his daughter, Emma frequently socializes with people from the same class. Among these are Mr. Knightley, Mr. Elton, and the Westons. People in this group are respectable and usually wealthy. The next class down of people are called "the second set." Mrs. and Miss Bates, Mrs. Goddard, and Harriet Smith are in this class. These people live in modest circumstances, but they still have a certain level of respectability. Below this group are those who do not usually mingle in polite society, such as laborers. Mr. Robert Martin, who is a farmer, is in this class. Emma notes that "the yeomanry are precisely the order of people with whom [she feels that she] can have nothing to do" (Emma, Chapter IV). People in this group are not poor and needy enough to require her charitable assistance. Farmers do not need her help, as they are able to support their families and live simple, contented lives.
Harriet Smith is described as "the natural daughter of somebody" (Chapter III), meaning that her parentage is unknown to most people. She is educated and has been raised with good breeding, which places her at the social level below Emma.
Harriet and Emma become friends, and the latter is determined to find the former a husband. Emma sets her eyes on Mr. Elton, who is a class above Harriet. Harriet expresses to her friend that she has an interest in Mr. Martin, the farmer. Emma steers her friend away from him, desiring her to marry up rather than down. On many occasions, Emma pressures Harriet to marry a man who is above her in social class. Emma eventually finds out that Mr. Elton has no interest in Harriet. She later thinks that Harriet has an interest in Mr. Frank Churchill. Harriet confesses that it is Mr. Knightley she admires. Emma realizes she has made it so that Harriet is only interested in men above her social class.
Eventually, the social classes become mixed when it is revealed that Mr. Frank Churchill is engaged to Jane Fairfax. Mr. Churchill is in a higher class than Jane. Emma finds out that Harriet had never given up her attraction to Mr. Martin. They become engaged and get married.
Jane Austen always wrote about social structure in her novels. In Emma, the title character is full of vanity. She places a great deal of emphasis on social structure. She pressures Harriet to marry above her class. This temporarily prevents Harriet from marrying the man she truly loves. Emma uses social structures as a key component in her matchmaking. This element of the story is important to the plot, which is why Jane Austen chose to use it.