“You shall not sacrifice to the Lord your God an ox or a sheep in which is a blemish, any defect whatever, for that is an abomination to the Lord your God” (Deuteronomy 17:1, ESV).
God instructed the Israelites to use the highest quality animals in their herds as sacrifices. If God didn’t designate any standards for the livestock, the Israelites would likely use the ones with blemishes or defects for sacrifices!
They would use the ones of lowest quality–the ones most convenient and practicable for them to use.
But God wanted the best livestock for sacrifices to Him, the ones that required a sacrifice on the part of the sacrificer.
Because the fidelity of the sacrifice indicates the fidelity of the sacrificer.