Final answer:
Nick Carraway attended Jay Gatsby's party because he was the neighbor and invitee of Gatsby, which gave him insight into Gatsby's world and the Jazz Age excesses.
Step-by-step explanation:
Nick Carraway attended the party because he was invited by his neighbor Jay Gatsby, who is the protagonist of The Great Gatsby. In the novel, parties at Gatsby's mansion are lavish affairs that attract a varied crowd of people, most of whom are not personally known to Gatsby. The parties serve as a backdrop for the unfolding drama between the characters and offer a glimpse into the excesses of the Jazz Age. Nick, being Gatsby's neighbor and part of the storyline that F. Scott Fitzgerald weaves, is naturally drawn into the extravagant and enigmatic world of Jay Gatsby. Consequently, Nick's attendance at the party is significant as it not only moves the plot forward but also provides the audience with an outside perspective on the complexities of Gatsby's life and the era.