Charles Ellicott Commentary Leviticus 4:29-31

"And he shall lay his hand upon the head of the sin-offering, and kill the sin-offering in the place of burnt-offering. And the priest shall take of the blood thereof with his finger, and put it upon the horns of the altar of burnt-offering; and all the blood thereof shall he pour out at the base of the altar. And all the fat thereof shall he take away, as the fat is taken away from off the sacrifice of peace-offerings; and the priest shall burn it upon the altar for a sweet savor unto Jehovah; and the priest shall make atonement for him, and he shall be forgiven." — Leviticus 4:29-31 (ASV)

And he shall lay. —The ritual prescribed in these verses is the same as that ordained in the case of the sin offering of the prince (Leviticus 4:24–26). In Leviticus 4:31, however, the phrase, for a sweet savour unto the Lord, is added to the burning of the fat pieces of the victim. This phrase does not occur at the sin offering of the high priest, the congregation, or the prince (compare Leviticus 4:10, Leviticus 4:19, and Leviticus 4:26), but is used at burnt offerings (Leviticus 1:9 and Leviticus 1:13) and peace offerings (Leviticus 3:5 and Leviticus 3:16).

It is supposed by some that these words are designedly used in connection with the least costly sin offering, to indicate that the humblest gift of the humblest person, if sincerely offered, is as acceptable to God as the most costly offering of the most exalted in the land.