In "Jabberwocky," by Lewis Carroll, what is the effect of the last stanza's being a repetition of the first stanza?
My first thought is that the repetition tells us that the story in the poem is over. Most of us are used to the convention that that hero returns home after her or his adventures, for example, and this return to the opening description similarly brings the poem full circle and signals its end.
Signaling the end of the poem in this way is probably all the more important because the poem contains so many "nonsense" words that we, as readers, need as much help as we can get in making sense of the poem. While Carroll is free to make up new nouns, verbs, adjectives, and adverbs, he doesn't (because, simply put, he can't) make up new prepositions or articles (such as the word "the"). Just as the nature of the English language limits his experimentation in this poem, he's perhaps limited by the conventions of storytelling.
I believe that the repetition of the first stanza at the end in Lewis Carroll's "nonsense" poem, "Jabberwocky," from his classic children's tale Alice in Wonderland, also is a restatement that all is right with the world, and that things go on despite the events that have happened previously. By repeating the first stanza, Carroll tells the reader that the deed is done, the Jabberwock has been slain, it is a time for celebration, and life will return to normal once again. "Twas brillig," and things are as they were before.