The emphasis on the use of the vernacular throughout the story reinforces the sense that it deals with people who are generally removed from more mainstream, polite society. The language of the of run-down, sordid little towns along the river, the devious king and duke, and of the African Americans all serve as a counterbalance to the more genteel, standard English that was used in most American writing previously. It gives a greater air of authenticity and vividness to the portrayal of those segments of society. This is the main effect it has on the reader, although those unfamiliar with such dialects may also find this aspect of the novel somewhat off-putting. 

This novel really marked something of a new departure in not only showcasing the use of American vernacular but also having, as a narrator, someone who didn't talk correct, formal English. Huck's dialect is not so marked as some of the characters he encounters, but it is still far enough removed from standard English to be quite notable, and he uses slang liberally throughout. In addition, he is still only a teenager; his voice is entirely informal, colloquial, and not in the least concerned with conforming to a standard usage of English. Indeed, being forced to use standard, polite English all the time has been one of his chief frustrations when living with the Widow Douglas, whose valiant attempts to civilize him don't achieve very much.

It has been observed that the use of the vernacular in this novel helped to develop an unmistakably American idiom in literature, and it is regarded as one of the most important American novels just in light of that fact.

 Many have complained about Huck Finn’s use of dialect, because it includes the N word.  The dialect is also pretty strong in some places and difficult to understand.

“Say, who is you? Whar is you? Dog my cats ef I didn' hear sumf'n. Well, I know what I's gwyne to do: I's gwyne to set down here and listen tell I hears it agin.” (ch 2, p. 8)

Yet Twain uses this dialect for a reason.  It was very important to him that he capture the way people actually talked.  He wanted to slave to sound like a slave, and the others to sound like whatever group he came from.

In fact, Twain explains in the beginning of the book in his “Explanatory” that there are many different dialects, in case “without it many readers would suppose that all these characters were trying to talk alike and not succeeding” (p. 5).  The reader, as a consequence, really feels like he or she is there. 

The dialects try to authentically capture what people sounded like.  This reinforces the setting of the book, in the pre-Civil War deep South.  It forces us to slow down, and become one with the story.