Among other things, the Old Testament sacrificial system was an object lesson to God’s people and to the nations that God is holy and that the sin of the people had offended that holiness. The animals sacrificed — as the type of what was to come — had to be without blemish. The perpetual shedding of the blood of substitutes served as constant reminder that God was continually holy, the people continually sinful. The promise, then, was for a once-for-all substitutionary sacrifice.
God’s indictment of his people in Malachi for offering polluted sacrifices — those that were lame, defective, sick — was not (primarily) because God needed meat and was getting less than he deserved. The people were due chastisement because their substandard sacrifice profaned the type, and muted the lesson it was intended to convey. If the people could offer blemished sacrifices, perhaps, then, the Messiah could also be blemished.
The severity of our sin and the holiness of our God require that the substitute be without stain. Our sacrifices and offerings (‘present your body a living and holy sacrifice’) must, therefore, also be the best we can muster, not because we satisfy God with our best — after all, our best is not good enough, and God has already been satisfied in Christ — but because even after we have beheld the Lamb who took away the sins of the world we need constant reminder of his holiness, and the perfection of his redemption.