All citation styles have a common purpose: to document the history of ideas. Each formal style—American Psychological Association (APA), Modern Language Association (MLA), Chicago—uses a different approach to achieve that goal driven by the history and focus of the scholarly community that produced the citation style. Chicago’s Notes and Bibliography (NB) style aims to keep in-text citations to a minimum for readability, for example, while APA style is focused on proper attribution of ideas to people in the main text itself.
Failure to follow some citation style in your writing will lead to accusations of theft of ideas—known as plagiarism—a very serious offense in communities where reputations and careers are built on the strength and originality of your ideas. Plagiarizing can prevent you from receiving an academic degree, lead to already awarded degrees being revoked, book deals being canceled, books being pulled from stores, and job loss, especially if your having been offered the job was based on a degree you received that is revoked.