In The Great Gatsby, Tom likes to show his wealth off. I'm answering your question based on how it originally appeared, when it read "affairs" instead of "affair." I take "affairs" to mean finacial goings on, like flaunting his polo ponies. The fact that he flaunts his wealth reveals his character. He is on the top of the food chain and likes people to know it. He sees himself as superior to others and is a part of the bourgeois, middle-class status quo.
Your original question also contained a question about the green light losing value to Gatsby once Gatsby had Daisy. The green light on Daisy's dock is a symbol of what Gatsby doesn't have, and of what he wants. It's his dream. It's what he longs for. That's why he stares at it. It's also a sign of how close Gatsby is to finally achieving his dream. He can actually see it across the bay.
If you're correct about the green light losing its significance, it would do so because Gatsby has the real thing, at least in part. Once he has Daisy, or thinks he does, he doesn't need to stare at the light anymore. He has what the light represents.