This is an obligation that like even fasting on Yom Kippur has exceptions for one's health if keeping this obligation.
Nevertheless, this is serious business that the Rabbis have struggled to interpret just like other serious issues. It does not give one license to replicate this behavior on any other day. A nice write up on this obligation can be read at My Jewish Learning.
As with many mitzvot, there is a bit of mystery as to why. Purim is the result of very foolish people doing very foolish things. There is evil running through the behavior and actions of each and every character, at least from a modern perspective, though we are told that Esther and Mordechai are above reproach. Modern eyes recoil at the slaughter of Haman's family, just as Modern eyes recoil at the story of Saul and his pity on Agag that lead to his rejection by God. People reason through this and come away literally horrified.
How could God sanction genocide? The people of Israel in their time recoiled as well, and spared the lives of Canaanites as well as others that they were commanded to destroy utterly. We look back on this with the knowledge that if these people had listened and faithfully followed these commandments they would have been spared much agony. A fact that is emphasized in the story of Purim in that Haman is a direct descendent of Agag and his existence and evil are the results of earlier "compassion and pity" only serving the purpose of providing an enemy the opportunity to attack another day.
Yet, today, this same compassion resonates with our hearts and just as Abraham and Moses struggled with God in the face of witnessing well deserved retribution, we find ourselves torn and even celebrating the humanity of those who "failed" to do as they were told. The Amalekites became the one enemy that must not be allowed to live because they attacked the weak and the vulnerable in a cruel and cowardly way. Saul is damned for his kindness in this matter. Though it is more accurate to say that Saul was damned for his behavior that resembled the Amalekites in that he killed the weak and vulnerable without remorse but spared that which would benefit him as spoils of war. The result of this action was to heighten and prolong the enmity between the people of Israel and the descendants of Amalek. So, why would God command that Saul attempt this act, knowing in advance that he would fail??? What does this say about God? Samuel? Kings?
Just like in so many teachings of the Holy Bible, all is not what it seems. Samuel chose Saul to be King because he looked like and acted like a King. God told Samuel to avoid Kings because they would be cruel and oppress the people, their spirits and ultimately bring ruin and despair to the people of Israel, but then after saying this allowed Samuel to do this very thing. Why?
Ultimately some come away from this conundrum with the feeling that insanity lay here. That ultimately the Torah leads one to dead ends and that God is just a work of fiction because the message is superficially inconsistent and frankly inhumane.
Yet, the people of Israel now look back on each act of kindness in the face of the demands of fanatical devotion and say to the Islamic enemy who does not question and struggle, and see that from the very beginning, from Abraham asking God to spare the people of Sodom and Gamorrah to Moses pleading for the children of Israel in the name of "What will Egypt think?"
Israel's role has never been to be a world power in the likeness of Rome, but rather to be a light to the people of the world. It is this insane ability to look at the eyes of suffering of those who are strangers and outsiders and see themselves that is the very purpose and meaning of so much of the suffering that Jews have accepted and survived over the ages. It gives rise to Hillel's admonition and summary of the Torah in the words.. "What is hateful to you, do not do to your fellow: this is the whole Torah; the rest is the explanation; go and learn"
So, it is in the utter stupidity of the drunk that we are called to experience and remember so that we might not look down our noses and forget that is behind this obligation. This observance is to stick out as odd, not normal. Yet, it also calls us to see through the eyes of those whom this is normal and remember so that we might be called to do the "right" thing even when it is not the "right" thing.
an obligation to get drunk? That is stupid. Kids too huh?