According to the Vilna Gaon, one of the reasons to include these unusually specific details is the Torah’s desire to demonstrate how grand an earthly party can be. The Mishnah in Avos (4:22) states that the pleasure of the future world is unimaginably greater than this world’s greatest pleasure. Without this description, how would we ever know how great this world’s pleasures can be?
In the view of the Dubno Maggid, the Jews who attended the feast did so on purpose. He quotes the Midrash (Esther Rabba 2:5) that portrays a dialogue at the feast wherein Achashverosh asks the Jews, “Is your G-d capable of doing more than this for you in the end of days?” The Jews respond with an obscure quote from Yeshayahu (64:3), “[Since forever, nobody has listened, nor heard,] nor eye has seen, Elokim, (זולתך) except for You, what You will do for [someone] who waits for You.” There being countless verses in TaNaCh that hint to descriptions of the World to Come, how is this verse the best answer to the question? To explain, the Dubno Maggid tells a famous parable describing a rich man with an angry wife. Throughout his day, she annoys him with constant bickering. Finally, a day comes when she has to take care of something out of town for a few days. On the first day of her absence, the man asks his servant to prepare a delicious meal and serve his best wine. He eats and drinks, and truly enjoys his moments of peace. He orders a similar bill of fare for the following day. Surprisingly though, the man’s wife returns from her trip early, just before his feast. She joins him for the meal and asks him afterwards if the food was as good the previous day. He answers, “Honestly the food was better today, but it was easier to enjoy yesterday when I could eat it in peace.” The Jewish response to Achashverosh is similar in that they hinted that their future reward will at least be superior in that it will be זולתך, which can also be translated as “without you,” meaning Achashverosh. For obvious reasons, they had to choose their words carefully so that their answer would not be taken as the insult it naturally was.