The comparison between slavery in ancient times and slavery in America during his time serves the purpose of highlighting the similarities and differences between the two systems. By drawing parallels between the two, the author aims to shed light on the inhumane treatment of slaves in both eras, while also emphasizing the unique aspects of American slavery, such as the racial component and the economic motivations behind it. Ultimately, the comparison serves to expose the injustice and cruelty of slavery, both in the past and present.