The primary purpose isn’t about the details of Marshall’s career. It doesn’t go into his prior jobs, his law school experience, other cases he might have worked on unrelated to Brown, etc. Although the passage discusses some of his work before Brown, that doesn’t make the overall purpose to discuss the details of his career before Brown.

The primary purpose isn’t about the effects of “separate but equal.” It’s about Marshall’s work overturning “separate but equal.”

This best captures the primary purpose, which is to tell us about the work that Marshall did before Brown that helped prepare the court to accept his argument against “separate but equal.”

The primary purpose isn’t to provide guidance to other litigators. The author never tries to draw any lessons from Marshall’s work and never directs any statements toward how readers might approach litigation strategy in other cases.

The primary purpose isn’t to call attention to the unsound nature of “separate but equal.” “Separate but equal” has already been overturned. It’s part of history, and the author doesn’t treat the doctrine as if anyone still needs to be convinced that it’s wrong. The passage focuses on the work that went into overturning it to support the claim that cases Marshall worked before Brown helped prepare the court to accept his argument. The author never tries to draw attention to the unsound nature of “separate but equal” and never tries to advocate against the doctrine.