Or HaChaim on Leviticus 23:22/אור החיים על ויקרא כ״ג:כ״ב

1 ובקצרכם את קציר ארצכם, "And when you reap the harvest of your land, etc." Why does the Torah introduce this kind of legislation at this juncture when it is totally out of context with the subject matter being legislated in this chapter? Perhaps the Torah wanted to inform us that the owner of a field from which the barley for the Omer sacrifice has already been taken is still subject to the variety of tithes the farmer is commanded to leave for the poor when he harvests his field. Had the Torah not written this I might have thought that inasmuch as this field had already become a sacrifice to G'd in that the Omer of barley had been offered from it on the altar it would no longer be subject to the legislation of the various donations which have to be separated from the harvest. Perhaps this is why only two of those gifts to the poor are mentioned here, i.e. פאה ולקט, the corner and the gleanings, both of which are duties which devolve on the actual field itself. This is different from the legislation of שכחה, already harvested sheaves, which the owner has forgotten to pick up. Our sages in Torat Kohanim say that the Torah alludes to the moral-ethical message that anyone who gives פאה, לקט, and שכחה is considered as if he had assured the continued existence of the Holy Temple and had offered sacrifices therein; thus far Torat Kohanim. The reason they mention the word שכחה although it does not even appear in our verse is because it is also a commandment similar to לקט ופאה. In this way they have hinted at what we have written above.