why does tom go to gatsby's party?

Tom attends the party in many ways to try and ruin Gatsby. Once there he is critical of everything about it, the decorations, the people that are there, the way that Gatsby behaves, anything he can be critical of, he does so. He also attempts to substantiate a rumor that Gatsby is a bootlegger and decides after the party that he will really dig into Gatsby's past and try to discredit him.

This event begins to unravel Gatsby's ascent into high society and starts the downward path to his destruction. It begins to become clear that Daisy's love for Gatsby is as false as her love for Tom and that the charade of high society is sadly a model for the love that Gatsby thinks he is going to find when he asks Daisy to abandon Tom and be at his side.