That is hard to understand, especially in our times. There is a lot of symbolism involved. The animals that were sacrificed were symbols of God giving his only son as a sacrifice for us. When people showed the willingness to give back to God their first-born unblemished male animals, that helped them to understand the gift that God would give to them, and showed their recognition and thankfulness. It would have been a big deal to the people in ancient times since the first unblemished male would otherwise probably be the most likely animal to be kept for future breeding. I've taken note in my flock which rams would have been sacrificed if we lived under that law, and they would have been the hardest to give up.

I think that we look at death differently now than they did back then too. Life is hard, and survival is hard, and for the animal, maybe going back to live with God was more of a blessing than staying for several years on earth. Especially when that animal was born to the purpose of sacrifice, and would have understood that before coming to earth (IMHO).

It wasn't so much for God's benefit as for the people to understand and remember that they needed to look to God for everything, particularly for redemption. Even understanding the purpose, I think it would have been very hard though, and I am glad that I don't live in Old Testament times.