In the fifth verse of Pareshat Kedoshim, the Torah notes that when a person brings an offering to the Beit Hamikdash, he should be sure that it is done “Lirtzonchem” –“willingly”. The Rabbis in the Talmud derive from this word that it is vital to have intention when performing the sacrificial service.
A few verses later, the text describes several gifts to the poor that are left in the field unintentionally. Despite the fact that the owner has no idea that he is sustaining the needy, it is considered a great Mitzvah.
What we are being told is that when we perform Mitzvot that are decrees that transcend rational reason, like the Korbanot, it is essential to be mindful of the commander, since we relate to them as doings that have only extrinsic value. Mindless irrational deeds are the core of paganism and its rituals, activities that the Torah warns us time and again to avoid.
Rational Mitzvot, on the other hand, have worth even when done inadvertently. If the poor person benefited from my field, good has been done.