The LORD told Moses to tell Aaron and his sons and everyone else the rules for offering sacrifices. He said:
The animals that are to be completely burned on the altar must have nothing wrong with them, or else I won’t accept them. Bulls or rams or goats are the animals to be used for these sacrifices.
When you offer a sacrifice to ask my blessing, there must be nothing wrong with the animal. This is true, whether the sacrifice is part of a promise or something you do voluntarily. Don’t offer an animal that is blind or injured or that has an infection or a skin disease. If one of your cattle or lambs has a leg that is longer or shorter than the others, you may offer it voluntarily, but not as part of a promise. As long as you live in this land, don’t offer an animal with injured testicles. And don’t bring me animals you bought from a foreigner. I won’t accept them, because they are no better than one that has something wrong with it. (Leviticus 22:17-25)
God does not run a thrift store. Those who give castoff clothing and other used items to the church do so because though they no longer wish to use the discarded items themselves, they believe that they could still be used by someone. However, God wanted his people to give up prized possessions when they gave him their stuff. In ancient Israel practically everyone was a farmer. Their animals were their wealth. Imagine setting your money aflame on an alter and you’ll have a good sense of what the animal sacrifices meant to the Israelites. Giving God the broken or damaged animals, the items that were otherwise unusable, would have been a natural temptation. After all, God was not obviously visible, not obviously there. Perhaps he was a long ways off. What would it matter what sort of thing was burned on the altar? Wasn’t it all symbolic anyhow? But that was precisely it: what mattered to God was the intent of the individual, the depth of his sacrifice, the meaning behind the offering.
Giving the best animals, the perfect animals, the most valuable animals, demonstrated a commitment to God. Why male animals? Because a single male could impregnate many females; the male represented future wealth. Farmers normally had many female animals, but only a handful of males: one bull, many cows, one ram, many sheep, one rooster, many hens.
God expects the best from us. Our sacrifices should cost us something.