While some students find it difficult to read different dialects, it can definitely enhance the reading experience. Twain, like others, had a very distinct voice in his writing, and in expressing his stories or characters in dialects he allowed his readers to become immersed in the setting, time, and lives of his characters. The reader is able to 'hear' the story unfold through the words and voice of the characters and narrator. Just as with a spoken dialect, writing in dialect helps establish many aspects of time, place, and character without added words; you are able to immediately envision Twain's world and the lively people who inhabit it, and are transported there to be enveloped in the story. Another example of a story written in dialect is Gary Paulsen's Nightjohn, a story told from the perspective of Sarny, an uneducated slave girl who wants to learn to read and write. Her words and dialect help place her in context, and allows the reader to see the contrast between the slave world and the more refined white world. It exemplifies the writer's credo of 'Show, don't Tell."
Well, Mark Twain is an excellent author to focus on to examine the stylistic use of dialect. In his works he makes a deliberate choice to try and use the vernacular (the langauges commonly spoken by people in a particular place or region), and this becomes a hallmark of Twain's style. The advantages of using the vernacular are that it creates a vivid writing style, as you capture the way that characters speak, and it also helps to evoke a particular time period or setting as the vernacular includes unique vocabulary, idioms and dialect. You need only to look at one of Twain's short stories such as "The Celebrated Jumping Frog of Calaveras County" to see the force and power of the use of the vernacular in creating unforgettable characters and situations.
However, obviously the use of vernacular can be difficult to understand if we are not familiar with it, which can be a challenge when reading passages containing large sections of vernacular.