In The Great Gatsby, Tom attends Gatsby's party because he does not want Daisy to go by herself. Fitzgerald writes,
"Tom was evidently perturbed at Daisy’s running around alone, for on the following Saturday night he came with her to Gatsby’s party.”
Tom also wants to see for himself how Gatsby and Daisy interact. He also wants to laud his position as Daisy’s husband and someone from old money over Gatsby. In this way, the scene reveals the contrast between Gatsby and Tom. There is something unpleasant about Tom being there, something almost menacing.
Nick detects a difference the night Tom attends the party. The champagne flows like always and the freeloaders are there chatting happily with one another, but there is a negative overhang. Nick writes that he “felt an unpleasantness in the air, a pervading harshness that hadn’t been there before.”
Importantly, Tom also wants to discover just who this Jay Gatsby really is. While Gatsby does everything he can to show off to Daisy and Tom, Tom is trying to uncover the truth about Gatsby, his background and the source of his wealth.
“Who is this Gatsby anyhow?” demanded Tom suddenly. “Some big bootlegger?”
"Where’d you hear that?" I inquired.
"I didn’t hear it. I imagined it. A lot of these newly rich people are just big bootleggers, you know."
Tom describing Gatsby as someone "newly rich" is his way of dismissing Gatsby and of making clear how far below Tom Gatsby is in the social hierarchy. Tom ends up saying, "I’d like to know who he is and what he does,’ insisted Tom. ‘And I think I’ll make a point of finding out.’
Tom also wants to mock Gatsby and point out how gauche his party and overall lifestyle is. Gatsby wants to show Tom and Daisy the number of famous Hollywood people at the party. He says, "You must see the faces of many people you’ve heard about," but Tom does not seem to be impressed.
Gatsby's pointing out that there are celebrities at his party gives him a sense of having arrived, having succeeded, being equal to people like Tom who have come from money and have not had to worry about it as Gatsby has.
However, Tom exudes arrogance at the party and clearly scoffs at Gatsby. According to Fitzgerald, “
Tom’s arrogant eyes roamed the crowd. We don’t go around very much,’ he said. ‘In fact I was just thinking I don’t know a soul here.”
Tom says this in an arrogant way aimed at deflating Gatsby. Tom does not care what famous riff-raff attends Gatsby’s party. To Tom, Gatsby is and always will be an arriviste.