why nick carraway is in west egg so we know about his character and bacgroundd

I'm not sure what exactly you are asking about. I will surmise that it has something to do with Nick's background and if it makes him a credible character. Nick is a pretty credible character. He doesn't have any eccentricities and seems grounded. He was educated at the Ivy School Yale and fought in WW1. Nick isn't a character who needs anything. He seems to have enough money to settle down in West Egg. He has come to New York for work and to learn the bond business; he doesn't seem to be in a hurry to get starte. For much of the novel Nick is more or less a flat character. We can see life in West Egg through him. Nick observes situations without giving judgment. This allows the reader to trust Nick as a narrator and finally a character in the novel.