A rule that forbids going back for what you dropped is training in how a person holds everything else.
What the hands practice becomes the setting of the mind. A man who leaves fallen grain where it lies teaches himself that there is enough, the belief loosens the grip, the loosened grip changes how he prices risk and how he treats people who need something from him, and those choices compound.
Yahweh attaches the blessing to the work of the hands rather than to the sheaf, which means the return comes through a changed man doing ordinary labor and not as a refund on what he left behind. Go back for every last stalk and the barn is fuller while the owner keeps the reflexes of someone who expects to run out, and that costs more every season.