Twain applies dialect here in the discussion between Roxy and Jasper to add an element of realism to the story.  By capturing the language actually spoken by Missouri slaves, Twain presents the characters as they really are.  The notion of realism, support by Henry James and William Dean Howells, prescribes writing which is true to life, with an emphasis on the common and ordinary.  For more information on realism, check the links section.