What can nineteenth-century scholars learn from the relationship revealed through the documents of Hannah Cullwick and Arthur Munby?

Hannah Cullwick was a working-class woman who worked primarily as a domestic servant. Arthur Munby was a middle-class lawyer and public official in London. Munby was also a researcher who collected information on working-class women, ranging from their clothing to their accents to the proportions of their bodies. He was viewed as somewhat odd by many of his contemporaries, but his "research," much of which seems to have dovetailed with his sexual proclivities, was used to inform public policy, including workplace and housing safety regulations.
After his death, it was discovered (through the correspondence mentioned in the question) that he had married Cullwick, having lived with her in secret for many years. First and foremost, their letters reveal a relationship between two people whose sexual preferences would have been viewed as deviant in Victorian London society. It seems that Munby was attracted to women who did not display the purity and gentleness expected of the model Victorian housewife, and Cullwick seems to have enjoyed her role in Munby's life.
There have been a number of books written about their relationship, including Watching Hannah: Sexuality and Bodily Deformation in Victorian England by Barry Reay, published in 2002, and Diane Atkinson's Love and Dirt: The Marriage of Arthur Munby and Hannah Cullwick. These books focus on the way gender and class intersected in Victorian England.
The fact that the dirt, grime, and physical strength that characterized Cullwick's appearance (which Munby so appreciated) underscores that working-class femininity was different from that of the middle classes that dominated popular culture. Cullwick and the other working-class women Munby knew were attractive to him because they were an "other," and Cullwick herself seems to have relished the role of domestic, at least on a performative level.
So in essence, their correspondence reveals the different layers and contradictions inherent in Victorian understandings of class, gender, and sexuality. For this reason, studies of this strange relationship feature prominently in historical discussions of gender and sexuality.
https://books.google.com/books?id=HUqgipzgR18C&printsec=frontcover&dq=Nobody's+Angels&hl=en&sa=X&ved=0ahUKEwj42PvokLjaAhXjmOAKHR7JBucQ6AEIKTAA

https://reviews.history.ac.uk/review/354a

https://www.theguardian.com/books/2003/jan/18/featuresreviews.guardianreview2