I understand why we are to drink wine on Purim, but why is it important to eat a special meal?
One reason is that the decree of Achashverosh and Haman was directed against the “body” of the Jewish People. There was no interest or attempt to uproot the Jews from Judaism and keep them alive, rather the intention was to annihilate the entire Jewish People – men, women and children. Therefore, the mitzva commemorating the salvation is through feasting in order to affirm the continuation of the Jewish People’s physical existence. This is in contradistinction to Chanuka where the Jews were in danger of spiritual annihilation, and therefore we light the menora to affirm our spiritual continuation.
Another reason is that feasting bears witness that the transgression of partaking of the forbidden feast in the days of Haman has been expiated. Otherwise, bodily affliction would have been prescribed rather than bodily pleasure. The Purim meal shows that we are free of sin regarding that feast, thus we are permitted to feast in order to commemorate the miracle of our salvation.
In addition, the Purim feast is especially significant in that it elevates the soul as it provides pleasure to the body. It is thus stated in the Zohar that on Purim one may accomplish the spiritual purification and elevation through feasting that he can accomplish on Yom Kippur through fasting.
The Purim meal is also reminiscent of the feast Ester held in order to expose the wicked Haman and thwart his evil decree. The Gaon of Vilna notes that for this reason the feast is held toward evening: The Megilla describes that Ester proclaimed a three day fast for the Jewish People to atone for their sins and to stir divine favor for her plan of exposing Haman. However, Ester did not fast the full third day but rather ended early in order to start the feast she prepared for Achashverosh and Haman. This is alluded to in the verse, “And I and my maidens will also fast thus”, where the Hebrew word for “thus” is “ken” (‘caf’, ‘nun’) which has the numerical value of 70. This indicates that while the Jews fasted a full 72 hours, Ester fasted only 70, after which time she commenced the feast toward evening which ours commemorates.
Although it is a mitzva to have an elaborate meal for this occasion, the expense of the meal should not be in place of distributing charity to the needy. However, one can fulfill both mitzvot simultaneously: The meal is considered all the richer if one hosts at his table the poor, the orphaned, the widowed and those who have no one to share the meal with. One who rejoices the hearts of these unfortunates is praised for walking in the way of G‑d, of Whom it is written, “He raises the spirits of the lowly, and restores the heart of the downtrodden” (Rambam, Megilla 2).
It is also very important to engage in Torah study and discussion prior to and during the meal as it is written in the Megilla (8:16), “Unto the Jews there was light and joy”. Regarding this verse the Sages commented (Megilla 16b) that “light” refers to Torah and “joy” refers to the pleasure of the meal. They thus instructed, “Let there be the light of Torah where there is the joy of the Purim feast”.