Sforno on Exodus 29:24/ספורנו על שמות כ״ט:כ״ד

1והנפת, this “waving” applies to the part of the sacrificial animal that the priests are given to eat. The main reason is that the priest should remain aware that he eats from שלחן גבוה, “from G’d’s table,” so to speak, i.e. that his portions are a “kick back,” totally voluntary gift from G’d, and that he does not eat by “rights,” i.e. that this is what the owner of the sacrificial animal designated for him. In this instance, also the part of the animal which would normally be his to eat wound up on the altar instead of being consumed by the priests, it being an introductory symbolic procedure. [perhaps similar to the concept of ראשית, the “first” part of crops which need to be consecrated to G’d of produce originating from the earth. Ed.]