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.