In Parshas Kedoshim, we learn about three commandments having to do with the harvest. We are first told that one should not harvest the corner of one's field (ויקרא י"ט:ט'); this is the Mitzvah of פאה, referring to the פאה, the corner, of the field. In the same Posuk we are also told not to gather the gleanings of our harvest; this is the Mitzvah of לקט, the gleanings. In the next Posuk (שם פסוק י'), we are told not to gather the fallen fruit of the vineyard; this is the Mitzvah of פרט, the fallen fruit.
With reference to פאה, Rashi says that not harvesting the corner of the field means to leave a corner at the end of one's field for the poor. Regarding לקט, Rashi says that if one or two stalks fall, one must leave them for the poor. However, if as many as three fall, that is not considered לקט and one need not leave them. And regarding פרט, Rashi says that the fallen fruit of the vineyard refers to single grapes, as opposed to the whole clusters. What is Rashi getting at with these remarks? Why these details about exactly how much fell and where they fell? Aren't these all just cases of accidental leftovers that go to the poor man and the stranger, as the Posuk tells us "לעני ולגר תעזב אותם," "to the poor man and the stranger shall you leave them?" What difference does it make how much fell? Furthermore, the Posuk (שם) ends with the words אני ה'" אלקיכם," "I am Hashem your G-d." Why the dramatic ending? Rashi explains that these words tell us that Hashem is the one who will exact punishment and will fight on behalf of the poor. But why are we so serious about all this, when we are talking about only a little charity from what one has in one's field?
Perhaps the answer is hinted to by Rashi's comment about לקט. Rashi says that if one or two stalks fall, that's לקט, but more than that isn't. Why? Maybe this is what the end of the Posuk comes to explain by saying "אני ה'," "I am Hashem." This means that Hashem is responsible for this so called accidental dropping of stalks. It's the person's accident, but it is supervised by Hashem. If more than that small amount falls, it can no longer be considered an accident, and the Torah specifically wanted the poor people to be provided for by means of an "accident." Why does Hashem want it this way? This is Hashem's way of building into the harvesting process a method to support the poor person and the stranger. One should not think that just because one planted something, he gets all the crops. Hashem will always provide ways to help others through the leftovers; this is why Hashem is ready to fight for the poor and stranger. After all, Hashem made these "accidents" happen, and thus for us to keep the leftovers would be a denial of Hashem's involvement in our world.