It wasn't optional, God required it as part of the law.
Exodus 29:10-14
"Then you shall bring the bull before the tent of meeting, and Aaron and his sons shall lay their hands on the head of the bull. "You shall slaughter the bull before the LORD at the doorway of the tent of meeting. "You shall take some of the blood of the bull and put it on the horns of the altar with your finger; and you shall pour out all the blood at the base of the altar
Numbers 6:14
'He shall present his offering to the LORD: one male lamb a year old without defect for a burnt offering and one ewe-lamb a year old without defect for a sin offering and one ram without defect for a peace offering,
Numbers 15:27
'Also if one person sins unintentionally, then he shall offer a one year old female goat for a sin offering.