How does the interaction between Uncle Hammer and the family help to move the plot forward?

Uncle Hammer and his Packard provides a marked parallel to Mr. Granger. On the surface level, Hammer provides a means for Cassie to see the possibilities available to black men beyond becoming farmers and workingmen like Papa and Mr. Morrison. The Packard is a mark of status, which symbolizes that Uncle Hammer is just as good as, or even better than any white man.

Hammer also provides a contrast to Papa, a man who is normally careful and thoughtful in his reactions. Hammer's knee-jerk reaction to Cassie's experience validates its injustice, but Mama's worries about what might happen to Hammer if he confronts Mr. Simms make it clear that even a black man of high status will not receive equal protection under Southern law.