What is the role of the gasoline advertisement in the narrative?

Vera leaves the apartment building and walks in the opposite direction of Bigger, towards the YWCA, but she decides to turn around and offer Bigger a final reminder that his next crime will send him to prison, rather than reform school. After Vera leaves, Gus arrives and he and Bigger lean against a wall to smoke cigarettes and watch an airplane sky-write an advertisement: USE SPEED GASOLINE. The plane represents the unattainable for Bigger. He will never have the money to fly or own a car. Bigger wishes that he could fly an airplane and after Gus sarcastically reminds Bigger of American racism, they are both depressed but they soon laugh at how naïve their "realizations" sound.