The poem uses a lot of parallelism (and, more specifically, anaphora) to build rhythm and momentum and also to draw attention to certain ideas.
Take a look at the first stanza. First of all, there's anaphora with the repetition of "This is" and "This is the voice of":
This is not a small voice
you hear this is a large
voice coming out of these cities.
This is the voice of LaTanya.
Kadesha. Shaniqua. This
is the voice of Antoine.
These repetitions are bold and insistent, much like the collective voice the speaker is describing. They also add a noticeable rhythm to the poem, giving it musicality and momentum, much like the powerful river the speaker uses to symbolize the voice of Black communities in lines 8-12 ("Running over waters [...] their river mouths.") The anaphora in lines 17, 24, and 25 ("This is a love") is again insistent, impressing upon the reader the importance that love plays in Black communities.
Parallelism can also draw attention to the connection (or contrast) between images and ideas. Take the phrase "This is not a small voice" vs. "this is a large voice." This is an example of antithesis, and the parallel structure of these two phrases adds emphasis to the speaker's point: that this voice is decidedly not soft and quiet, but rather loud and bold.
Meanwhile, the parallelism of the phrase "This is not a small love / you hear" at the beginning of stanza 2, which mirrors the lines "This is not a small voice / you hear" at the beginnings of stanzas 1 and 3, suggests that the "voice" and "love" of Black communities are inextricably intertwined.