Explain why plebeians were so important to Rome?

Read:
Chapters 6 through 13 (Morey, W. C. (1901). Outlines of Roman history. Forum Romanum. http://www.forumromanum.org/history/index.html)
Watch the video: Arzamas. (2017, May 30). Ancient Rome in 20 minutes [Video]. YouTube.

For the paper, answer the question:
Why were plebeians so important to Rome?

Be sure to use in-text citation and provide references for your sources, including textbooks.