The last line is like a punch because Fitzgerald gives us emphasis by giving sound to the sentence. So we beat on, boats against the current, borne back . We all keep getting stuck in the past. The line, too, gives the whole novel an extra bittersweet moment. We are there in the past of the story and the present of our lives.
Life is both bleak and beautiful.
Alliteration This is what Fitzgerald used in the last line above with the B's. It is the repetition of initial sounds of stressed syllables. The close juxtaposition of vowel sounds creates assonance. Euphony Lines that are musically pleasant to the ear bring euphony. There is a harmony and a beauty to the language, which is what many poets are often after.