Why does Fitzgerald describe the party ( in the passage beginning "By seven o'clock the orchestra has arrived") in the present tense?
This is a curious passage. The first three paragraphs of chapter III are written in the past tense, telling us in a general way through Nick's eyes what Gatsby's parties are like as repeated performances. Then the narration moves to three paragraphs written in the present tense, beginning with "by seven o'clock," and finally reverts back to the past tense as Nick narrates being invited to and attending a Gatsby party.
Present tense provides an immediacy that the past tense doesn't. For three paragraphs, we are put at a particular party on the cusp of beginning. We are witnesses at the scene as the action unfolds. We feel the excitement of this party as it starts, and while we "know" Nick is telling this story, these three paragraphs are largely unmediated. We are allowed to glimpse, at least for a moment, how exciting these parties are.
The three paragraphs provide a prelude and backdrop to Nick's story. The shift in tense to past indicates that Nick's particular saga is now beginning.
The effect of describing one of Gatsby's parties in the present tense is to create the impression that the same thing is happening over and over. If you've seen one you've seen them all. Every one of these parties is the same, so it is sufficient to describe on of them to create a sort of montage effect representing all of them. There is a certain feeling of monotony created by this present-tense narration and description. The parties are supposed to be glamorous and exciting, but they are really meaningless rituals, like a lot of things in the Jazz Age. People are spending a lot of money, trying to have a good time, and pretending to have a good time, but just getting drunk and exhausted and acting like fools. Gatsby's parties have a feeling similar to Edgar Allan Poe's "Masque of the Red Death." F. Scott Fitzgerald's descriptions of these wild parties is brilliant. He seems wise beyond his years, not unlike Nick Carraway.
So we see exactly what Nick can see at the time and we experience the whole party with him, it is almost as if we are there with Nick moving around in the hazy confusion and fast pace movement together. We get a real sense of the situation he is in and fully understand what one of Gatsby's famous parties are like.
Hope this helped.