There are two reasons to use present tense verbs. The first is to express things that are happening right now in the present, and the second reason is express habitual actions or statements of fact. In this case, in the descriptions of the party in Chapter 3, the effect of the switch from past tense to present tense intensifies the idea that these things that are happening are what always happens at Gatsby's parties or in preparation for them. Nick is, for the most part, flashing back to his attendance at this party, but he moves in and out of past tense to make it clear that these parties, the preparations, the types of guests, and the activities, are always the same. The present tense helps to capture the atmosphere of the parties. The desciption suggests that the parties are beautiful, grand, elegant and almost magical. For example, Nick tells that "the lights grow brighter as the earth lurches away from the sun." This is a great example of the hyperbole of all aspects of Gatsby's parties.
Fitzgerald describes the party in the present tense because he wants the reader to feel like a guest at Gatsby's house. Specifically, to experience the party first hand and to understand what attending such an event might have felt like. The present tense provides Fitzgerald with the most effective way of doing this: it enables the reader to clearly picture the sights, sounds and smells of such an event. It is, therefore, far more effective than having Nick describe the party from his memory.
This particular party from Chapter Three is significant since it is the first one which Nick attends and which the reader experiences. It is crucial, therefore, in helping the reader to understand Gatsby's world and this is another reason why Fitzgerald writes in the present tense.