The single-sentence structure of Jamaica Kincaid's "Girl" creates the impression of a deluge of advice delivered at great speed. There is a hectoring quality as the speaker bombards the girl with advice, rattling off commands like a machine gun. However, the reader may also detect an insecurity in this speed: perhaps the girl is inclined to run away and disregard all this advice, so the speaker pours it out in vast quantities whenever she has the chance.
Punctuation gives structure to a sentence, separating it off into manageable clauses. Even more significantly, it separates ideas into sentences. Readers are so used to this that, when a piece of writing takes the form of a stream of consciousness, without periods, they automatically seek other systems of organization. There are plenty of these in "Girl," principally the use of anaphora and other types of repetition. At one point, the words "this is how you" are repeated twelve times. This would be striking in any context, but is emphasized by the single-sentence structure.
Finally, the structure gives the piece an improvised quality, as though the speaker is saying the first things that come into her mind, regardless of their significance or connection. This leads to interesting juxtapositions, and allows major themes, such as singing benna and talking to men, to be revisited at regular intervals.