There are many meanings to the parties in the Great Gatsby. Let me give you a few of them.
First, the great parties are a commentary on the times in which Jay Gatsby lived. It was a time of unprecedented wealth in America. The "roaring twenties" were a time of conspicuous spending and wealth. The fact that the novel does not end on a happy note shows that money cannot buy happiness.
Second, there is also a note of autobiography in these parties, as Fitzgerald himself was known to lead a wild life of partying. He also tried to win a girl's heart in the process, much like Jay Gatsby.
Third, from a literary point of view, the parties are over the top and beyond excessive. So, from this point, Jay Gatsby lives up to the title of the book, "The Great Gatsby." He is like an illusionist or performer who can bend reality and create something from nothing. It is about showmanship. The tragedy is that it is all an illusion, and in the end nothing will come of it.
Finally, these parties serve to heighten tragedy in that all these friendships are fickle. When Jay dies, no one is there except Nick. Big parties, lots of people, but they are users. These parties, therefore, show the hollowness of the people.