Miss Bates becomes a sort of horror story in the novel. Well, maybe that’s not exactly fair. After all, Miss Bates is sweet, generous, kind, and thoughtful. She might like gossip a bit too much, but then again, so does just about everybody in Highbury.
When we say that Miss Bates is a walking horror story, then, we mean that she’s the novel’s poster child for why women need to get married. Miss Bates grew up as a gentlewoman, provided for by her father and well-respected by all of Highbury. When her father and brother die, however, Miss Bates has no one to support her – or her mother, for that matter. When Jane arrives, things get even worse – which might be why Jane was sent to live with her father’s friends. By the time the novel starts, the Misses Bates (mom and daughter) live largely off the charity of others. It’s a hard life for them, especially since Miss Bates can remember a time when everybody – including Emma – looked up to her. Now Emma makes fun of her at parties.
How do we know that it’s bad to make fun of Miss Bates? Well, as your kindergarten teacher probably told you, it’s mean to make fun of other people. More importantly, however, our narrator tells us early on in the novel that no one should say mean things about Miss Bates: "[…] she was a happy woman, and a woman whom no one named without good-will. It was her own universal good-will and contented temper which worked such wonders." In other words, Emma’s not just breaking the rules of social convention. She’s also breaking the express dictates of our narrator herself. And nobody messes with the narrator.
Fortunately, Miss Bates actually does have a heart of gold. She may not say intelligent things, but she’s more than willing to forgive anyone who slights her – including Emma. And she’s always happy to hang out with Mr. Woodhouse, which makes her the perfect friend to keep around during dinner parties. If she realizes that she’s got a rough life, we never hear about it.
Besides providing comic relief, then, Miss Bates serves a couple necessary functions in the novel. She actually proves that people like hanging out with Mr. Woodhouse. She also provides an implicit rationale for Jane becoming a governess: sure, taking care of kids might be awful, but wouldn’t it be worse to end up like Miss Bates? Austen loves to pepper her novel with characters that do double-duty as characters in their own right and symbolic warning signs for others. Emma herself recognizes Miss Bates’ role as the pitiful unmarried woman; she’s quick to point out that her life would be different from Miss Bates’ because she has money that will last past her father’s death.