And in Grease, cars symbolize both the era in which the film is set, and the private lives of the individual car owners. Kenickie's car is in bad shape, which shows us that he isn't from a wealthy family. He has to work, even for that hunk of junk.
By contrast, Rizzo's pink Studebaker Commander is a slick ride. We never hear her talk about a job. Could her parents have bought her this car? Is rough around the edges Rizzo really just rebelling against her wealthy family? A car says a lot about its driver.
With the help of his friends—and Mrs. Murdock—Kenickie turns his rust wagon in Greased Lightnin. Although his car wins the race, Kenickie isn't the driver. But maybe he'll get to keep Leo's car? Maybe that's why Rizzo respects Kenickie more at the end: two half-wrecked cars are better than one totaled junk pile.