If this law were in isolation, it might be irrelevant to all but historians of the ancient world. But no law in the Old Testament is in isolation. In the Law of Moses, meaning all the laws revealed at Mount Sinai to the new nation of Israel, the many laws come together under the the great “words”: what we call the ten commandments. Just as the first chapter of Genesis begins by dividing between day and night, the waters above and below the sky, and the land and seas, and then the rest of the chapter populates them (sun and moon, fish, birds, etc.), so some people have seen the ten commandments as dividing human activity into ten spaces, which the rest of the laws then populate.
Many readers of the Bible over the centuries have come to see each individual law as fleshing out one of the ten commandments: so every law has a context and a role within that context. For example, one ancient rabbinic discussion of the law of gleaning appears to relate it to the commandment not to steal:
One who does not allow the poor to glean, or who allows one and not another, or who helps one of them — he is stealing from the poor.
The reasoning is straightforward: the land itself belongs to God, and it is his to allocate the harvest. The bulk of the harvest he allocates to the farmer, but the edges and leftovers he allocates to the poor. Therefore, if the farmer were to take to himself what God allocated to the poor, the farmer would be stealing from the poor.
Suddenly, this law about gleaning takes on new life. It only makes sense in a world in which things we assume belong to us (like land) do not actually belong to us. Some like to talk about being “stewards” and being responsible to God for what they do with their belongings, but this still assumes that their “belongings” actually belong to them. Surely I am free to do as I like with what belongs to me? If I make good decisions, that will make me a good steward?
In the context of this law on gleaning, doing what we like with our “belongings” might actually constitute stealing. We tend to think of meeting our own needs first, and then, if there is anything left over, we can consider giving to others. (That is, if our needs don’t grow along with our means!) But consider: every field would have its edges and gleanings, regardless of its size and how bountiful its crop. And every farmer would have his obligation to remember those poorer than himself. Nobody with a field is exempt from remembering the poor. Instead of “meeting the budget” first, and giving away the surplus to the poor, the portion of the poor is part of the budget in the first place.
From the angle of the poor, this can be rather surprising. God has already provided for them, in what he has given the community. Even if they have no land of their own, in an agrarian community, there is no need for the poor to be destitute. God has already allocated them food, if only the farmers will not get in the way of letting them get to their food. It is not the farmer’s responsibility to take the food to the poor; the poor must come and claim it. But it is the farmer’s responsibility to refrain from taking to himself what rightfully belongs to the poor.
This is not redistribution with the aim of eliminating socio-economic differences. That is simply not the point of this passage. This is caring for the poor, regardless of how they became poor, and enabling them to survive. Gleaning would not provide amply for them, but it would provide. The poorest ancient Israelite should never have had to worry about survival, if the laws were followed. And every single farmer had a choice to make, every time he harvested a field: would he maximise his profit or would he remember the poor, and not steal what God had allocated to them?