Answer:
Because the Valley of Ashes is a train route connecting the East egg to West Egg in New York
Explanation:
The train always halts for about half a minute at the Valley of Ashes because the Valley of Ashes is a train stop, an important connection point between East and West Egg.
Tom mistress, Myrtle lives with her husband above a gas station in the Valley of Ashes. Hence on their way to New York, Tom insists that Nick gets off the train to get to see his mistress.
The Valley of Ashes is a strategic meeting point because it symbolises the persons involved - both Nick and Tom are nouveau-rich upper class people who usually transit through the Valley of Ashes, a symbol of hopelessness,neglect and poverty, to exploit the poor people there.
Myrtle on the other hand, even though she lives in the Valley of Ashes, dreams of living a life of affluence and power and the meeting with Tom is the only way she can live that dream of affluence even though it is at the expense of her husband and marriage. She, however doesn't seem to care about them both.
