What is the authors likely purpose for including parenthesis ellipses and dashes in paragraph 28

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in railroad songs, workers swung their hammers during the offbeat. work songs combined with traditional musical and lyrical forms, such as hymns and ballads, to create spirituals, the blues, and other jazz styles. this blending of old and new can be seen when comparing the lyrics to the traditional irish song "lass of roch royal" and the blues song "the deal."

as work songs and other forms of jazz developed, jazz music spread across the united states. for example, a jazz style known as dixieland went from new orleans to chicago by means of the famous trumpet player louis armstrong. the original dixieland band developed out of armstrong's influence and, in the early 1920s, moved to new york city. once jazz reached new york, especially harlem, its development