What is the effect of listing different types of workers and what they are doing in i hear America singing

It illustrates how the worker's unique songs create harmony.

The song for each worker is special, because it is his and no one else's. His song is what makes each one a great worker. It is a poem about masons, carpenters, working mothers, plows, mechanics and boatmen who meet to sing about their work. At the end of the day the workers gather as one song and celebrate.