Not many people in the novel have cars. In fact, most of the poor black families get around by foot or with a horse and wagon. The Berrys are lucky enough to have an old Model T Ford, but other than that, the only local people to own cars are Mr. Granger (fancy Packard) and the Wallaces (ominous pick-up truck that shows up in the dead of night).
So what does all this horsepower mean?
Cars mean power—the power of independence. For example, Big Ma tells Little Man that one day he'll have "plenty of clothes and maybe even a car of [his] own to ride around in" (3.12). So, it's definitely something to aspire to.
But cars also have another kind of power: the power to inspire terror. The night men don't trot up on horses; they come roaring up into people's yards in the dead of night with their headlights blazing.
As any teenager—or, you know, adult—knows, cars are also the ultimate status symbol. Check out Mr. Granger's Packard:
Soon the purr of a motor came closer and Mr. Granger's sleek silver Packard eased into view. It was a grand car with chrome shining even in the rain, and the only one like it in the county, so it was said. (3.26)
This ride just drips wealth and opulence (it's "sleek" and "silver"), and shows off Mr. Granger's wealth. Uncle Hammer has the exact same car (only newer, so neener-neener!). He uses it as a mode of defiance when he guns it across Soldier's Bridge before letting the white family cross first (6.164)—and that's got to feel good.