Gatsby is portrayed consistently as a lonely, isolated man. He is actually quite a shy person, as is seen when he first meets Daisy at Nick’s cottage. He only hosts these lavish parties in the hopes of attracting Daisy to one of them and having an opportunity to renew their relationship. Here again he shows his shyness, because he might have simply called her on the phone or sent her a letter. He is awkward in social situations. He would not enjoy drinking and participating in the antics of his inebriated guests. He is a dreamer—and as such he needs privacy and quiet. Fitzgerald would have had a very hard time of showing Gatsby socializing with dozens of guests at a party, so he avoided it by having Gatsby generally keep his distance. His reluctance to mingle was one of the factors that made him such a man of mystery. Furthermore, he seemed to be in constant telephone communication with agents all over the United States. If he had tried to mingle at his parties, he would have been called away time after time to handle some problem in a distant city. Fitzgerald invented all these phone calls because there was a question by his editors about how Gatsby was managing to make all his millions without seeming to go very far from his big private mansion. He is rather implausible as a gentleman mobster—but Fitzgerald was a very young man and did not really know much about the underworld. Fitzgerald was more comfortable describing wild parties than the activities of criminals.
While this question may have many correct answers, the most relevant answer is that he is not throwing the parties to mingle with guests at all. Well, he is, but only one guest -- Daisy Buchanan.
The parties are to function as a beacon for Daisy, drawing her in. As she lives in a rather rich and affluent circle, Gatsby must put himself forward as a rich and affluent man. Beyond that, though, he wishes to meet Daisy again, and to present himself in a way that it seems that he could give Daisy everything that she could ever want.
This is why he throws such extravagant parties, not to hobnob with the upper crust, but to put himself in a position that he may once again meet Daisy, and to meet her in a way that flaunts his wealth, so that she would want to spend the rest of her life with him.
Gatsby is not a huge fan of parties, in fact, he simply does not enjoy them at all. He threw the lavish party with the hopes that Daisy would come.