The giant billboard brooding over the Valley of Ashes isn't just a large advert; it's a hugely important symbol in the story. The symbol of the billboard can be interpreted in a number of different ways. Its rain-sodden, dilapidated state can be seen as a tart commentary on the tawdry commercialization of American life during the Roaring Twenties. At the same time, it acts as a reminder of the loss of spiritual values, which the unhealthy obsession with the headlong pursuit of riches has brought about.

In the absence of old-fashioned values, the all-seeing eyes of Dr. Eckleburg take the place of a transcendent law-giver, looking down sternly upon those who inhabit and pass through the Valley of Ashes, subjecting them to its God-like gaze. That's how George Wilson, going out of his mind with grief at the death of Myrtle, sees them. He explicitly equates the eyes of T.J. Eckleburg with the eyes of God. Mr. Michealis tries to point out to him that it's just a billboard, but George—and the reader—knows differently.

The billboard advertising Dr. TJ Eckleberg over the Valley of Ashes is not described as a billboard because it has a much greater significance in the novel than just that of an advertisement for an eye doctor.  The eyes that are present in the billboard are symbolic of a god-like figure, even God, looking over the Valley of Ashes and all corrupt events that take place in the novel.  If Nick had described it as simply a billboard, its significance would be lost but since he describes it as a pair of eyes, the reader associates vision or someone looking out at everything.