As the only African-American man on the ranch, Crooks is subject to treatment that all the other men working with him aren't. He is often called a "n****r" by Curley and the rest of his peers. He stays in a room separate from the rest of the ranch workers (who are all white). During an encounter with Lennie (in which Lennie states that he doesn't understand why Crooks is treated differently), Crooks is forced to explain why he remains separate from the majority of those on the farm and why he's grown to like it that way. While he is only in the novel for a short amount of time, Crooks' powerful testimony about the racism on the ranch aids the reader in understanding that even among working-class people, there are still strong prejudices held against those who are different. This interaction also helps connect the mistreatment he faces with the mistreatment that Lennie faces.
[P1] Crooks is segregated and mistreated on the Ranch. [P2] Crooks' mistreatment on the ranch serves as a testament to how prejudice exists, even among the working-class.