Why do you think this clause was important to the barons?

To emphasize the persistence of racism.

Explanation: Indeed, his descriptions of slavery were used to demonstrate several things; but the most important was that racism was a byproduct of slavery and that it sought to perpetuate it through other forms of oppression. A recurrent argument of Southern white supremacists was that since slavery was over, African Americans now enjoyed equal rights and that segregation ensured such “freedom”. By reminding his followers and listeners that slavery was still perpetuated by the subjugation, segregation, and imprisonment of African Americans.