Why Martin Luther king used parallelism

Parallelism is a rhetorical device that involves the repetition of words, phrases, or sentences that have the same grammatical construction. Here is an example from Martin Luther King's famous "I Have a Dream" speech, given during the March on Washington in 1963:

"I have a dream that one day even the state of Mississippi, a state sweltering with the heat of injustice, sweltering with the heat of oppression, will be transformed into an oasis of freedom and justice."

In this example, "sweltering with" is repeated twice, followed by "the heat of" and then a noun. Dr. King repeats the word "sweltering" to emphasize not only the heat of Mississippi but also its racism and injustice. Dr. King also included several paragraphs in his speech that began with "I have a dream," and he repeated not only the grammatical construction of the sentence but also entire phrases word for word.
Dr. King used parallelism in part because it was a common rhetorical device for ministers, who were used to presenting their sermons orally. Parallelism helps listeners take in the message of the sermon through the use of repetition. In addition, parallelism has a lyrical, musical quality that makes the words sound more pleasing and therefore more memorable. Dr. King used this device to emphasize his dreams and make sure the audience absorbed them.
https://www.archives.gov/files/press/exhibits/dream-speech.pdf