"When you reap the harvest of your field and you forget a sheaf in the field, do not return to take it but let it be for the proselyte, the orphan and the widow."
When a Jew is harvesting his field or the fruit of his trees, he must not retrieve what he has forgotten in the process. There are laws governing what constitutes forgetting which are detailed and clarified in Mesechta Peiah.
The needy set their eyes on the produce of the field as the sheaves or fruits are being reaped and bound and they think in their hearts how wonderful it would be if they could have some of those crops. It was therefore an expression of G-ds loving-kindness toward His creations to fulfill this yearning when the owner of the field forgets it.
The benefit for the owner is that he will thus develop a generous character and a blessed spirit for it is certainly a quality of generosity and a blessed spirit not to care about a forgotten item of produce and to leave it for the needy.