The Lord commanded certain offerings, but He also knew that the Israelites would want to give freely at other times, nevertheless, He still regulated those offerings,
And the Lord spoke to Moses, saying,
“Speak to Aaron and his sons, and to all the children of Israel, and say to them: ‘Whatever man of the house of Israel, or of the strangers in Israel, who offers his sacrifice for any of his vows or for any of his freewill offerings, which they offer to the Lord as a burnt offering—you shall offer of your own free will a male without blemish from the cattle, from the sheep, or from the goats. Whatever has a defect, you shall not offer, for it shall not be acceptable on your behalf…” (Leviticus 22:17–25).
Any offering somehow represented the future offering of Jesus Christ, who was spotless before God, never having sinned, and so the Israelites, perhaps without realizing it, had to offer things that kept up that theme of perfection.