"The Real Thing" is a short story by Henry James. As a work of fiction, it doesn't have a "purpose" in the sense a persuasive essay does of trying to persuade someone of something, nor do we have the ability to read James' mind to discover what he was thinking as he composed the story. We do know that James was a professional writer and earned his living from his writing. Thus, one of his motives for writing was simply to earn money.
The story itself is a sort of parable about the nature of art. While James considered himself a realist in representing the details of life as it actually happens, he also argues in his essay, "The Art of Fiction", that literary works are reality as it is filtered through a writer's sensibility and crafted to convey an impression to a reader. In this essay, James also sets out many parallels between writing and painting.
In "The Real Thing", an artist is approached by Major and Mrs. Monarch who want to work as models. They are a well-bred couple who are actually the sort of people who appear in the artist's portraits. The artist's current models are a cockney girl and Italian vagrant. When the artist tries to use the Manners as models though, he discovers that they are worse at modeling than professional models who are not "the real thing". Here, James is suggesting, through an analogy with painting, the need for artifice and craft in all forms of art. Thus the story suggests that a writer simply transcribing everyday conversation would not produce realistic dialogue, but something awkward and aesthetically lacking, just as the artist fails to produce good work when he uses "the real thing" instead of professional models.