Adam Clarke on Exodus 29:10

Shall put their hands upon the head of the bullock - By this rite the animal was consecrated to God, and was then proper to be offered in sacrifice. Imposition of hands also signified that they offered the life of this animal as an atonement for their sins, and to redeem their lives from that death which, through their sinfulness, they had deserved. In the case of the sin-offering and trespass-offering, the person who brought the sacrifice placed his hands on the head of the animal between the horns, and confessed his sin over the sin-offering, and his trespass over the trespass-offering, saying, "I have sinned, I have done iniquity; I have trespassed, and have done thus and thus; and do return by repentance before thee, and with this I make atonement." Then the animal was considered as vicariously bearing the sins of the person who brought it - Exo 29:14