Parallelism:
Also referred to as parallel construction, or parallel structure, this term comes from the Greek roots meaning "beside each other". It refers to the grammatical framing of words, phrases, sentences, or paragraphs to give structural similarity. This can involve repetition of a grammatical element such as preposition or verbal phrase.

Example:
I have a dream that one day this nation will rise up and live out the true meaning of its creed: "We hold these truths to be self-evident: that all men are created equal."
I have a dream that one day on the red hills of Georgia the sons of former slaves and the sons of former slave owners will be able to sit down together at a table of brotherhood.
I have a dream that one day even the state of Mississippi, a desert state, sweltering with the heat of injustice and oppression, will be transformed into an oasis of freedom and justice.
I have a dream that my four children will one day live in a nation where they will not be judged by the color of their skin but by the content of their character.
-Martin Luther King Jr, I Have A Dream Speech

Explanation:
MLK Jr. uses parallelism in his repetition of "I have a dream". The purpose of this is to emphasizes his point that he wants equal rights for African Americans, and that he sees this in their future. The line "I have a dream" is also used because it can relate to almost anyone listening to the speech, everyone thinks of the future, and how to make it better, which is what MLK was trying to do for African Americans. Lastly, it creates a rhythm and balance that lets MLK expresses his views concisely and smoothly.