Like K.'s sexual shenanigans, art provides one of the seeming digressions in The Trial that is actually a critical part of the whole story. In addition to the central discussion of painting in Chapter 8, numerous references to the theater and to literature throughout the text indicate the novel's own attempt to wrestle with the fact that it is itself a work of fiction, a work of art. In the novel, all of these artistic artifacts fascinate and transfix the characters. They demand the characters' attention, and the characters can't help getting lost in their images. The power of art over the characters' minds and emotions parallels the court's equal power to fascinate and attract, as K.'s inability to resist being absorbed into the court's system attests. But the flip side is that art can also provide a way for characters to gain mastery over their situation, as when K. acts out the drama of his interrogation for another character. Art isn't just a fancy way to dress up a wall in the novel; it's a way of making sense of the world, of arranging your impressions of the world in a way that makes sense to you. Thus art, and by extension Kafka's novel, can provide access to certain truths about life that are otherwise inaccessible.
Art in The Trial serves the function of creating meaning out of the apparent senselessness and absurdity of existence.
The Trial demonstrates the futility of individual effort by showing how all of K.'s attempts to influence his case are reduced to mere theatrics.