Why do you think Fitzgerald includes the scene at Wilson’s garage at this point in the novel?

This scene is the flash point of the chapter. The Valley of Ashes becomes a space outside the realm of the super rich. The valley is an alien-like place that does not conform to the will of any of the characters. It is here that the chaos takes place; it is here where chaos of their social lives is accounted for. Mildred runs out into the street and the two worlds literally and figuratively collide.