Why does Nick go through the trouble of listing so many of Gatsby's party guests?
There are at least two factors involved here; there's Nick the character, and Nick the narrator of the story. That is to say, Nick the character may have his own reasons for making a list, but Fitzgerald probably had his own, separate reasons, and he's just using Nick to get them across.
Nick the character comes across as proud, but a bit depressed and put-off at the life of luxury, represented by Gatsby, being just out of reach. Yet as much as Nick condemns the hedonistic lifestyle and everything it stands for, he craves it very much. This is evidenced by the strange dichotomy of his house being an eyesore, but being in "consoling proximity" to millionaires. Likewise, Nick's list is, in a sense, consoling; it's an index of all the millionaires that he's in proximity to.
At the same time, Nick seems to make frequent and negative commentaries on the list, regarding the misfortunes or bad behaviors of the guests. Here Nick the character and the narrator overlap somewhat; this may be Nick's inner distaste and growing judgmentalism revealing itself, or it may be Fitzgerald's commentary on this class of society and the low moral standards that their celebrity disguises. The date may also be important from the narrative perspective; July 5th, the day after Independence Day. It might suggest that "this", i.e. the list of people, is what the nation has grown into; it is an ironic commentary on how far the country has fallen from the lofty principles it was founded upon.
Nick's negative comments might also be an indicator of his outsider status; not being part of this group, and therefore supposedly unable to appreciate them for their names alone, he reveals himself by needing to "spice up" the list with scandalous details. Nick might be just as bored with a list of unrecognizable names as we are.