In Genesis 8:20, Noah creates an altar after getting off the Ark:
Gen 8:20 ASV - 20 And Noah builded an altar unto Jehovah, and took of every clean beast, and of every clean bird, and offered burnt-offerings on the altar.
Why did he do this?
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A common error people make is to understand the sacrifice of animals in the scripture as a "payment" for sin as if somehow the suffering and death of the animal gives God consolation. This reflects a completely pagan notion of god:
Isa 1:11 ASV - 11 What unto me is the multitude of your sacrifices? saith Jehovah: I have had enough of the burnt-offerings of rams, and the fat of fed beasts; and I delight not in the blood of bullocks, or of lambs, or of he-goats.
These are properly understood to be symbolic of deeper realities and gestures of the inner person and apart from being an expression of a right heart, abominable:
Isa 66:3 NLT - 3 But those who choose their own ways--delighting in their detestable sins--will not have their offerings accepted. When such people sacrifice a bull, it is no more acceptable than a human sacrifice. When they sacrifice a lamb, it's as though they had sacrificed a dog! When they bring an offering of grain, they might as well offer the blood of a pig. When they burn frankincense, it's as if they had blessed an idol.
The most prominent gesture made by the sacrifices under the Torah were animals offered to God as expressions of contrition and an appeal to God for forgiveness:
Exo 30:10 KJV - 10 And Aaron shall make an atonement upon the horns of it once in a year with the blood of the sin offering of atonements: once in the year shall he make atonement upon it throughout your generations: it is most holy unto the LORD.
But there were also sacrifices offered in gratitude. Unlike those killed as sin offerings these could be eaten, resulting in a celebratory BBQ picnic for the Levites.
There was sacrifice also involved in the dedication of priests and the altar as well.
Since the occasion of Noah's sacrifice was the safe deliverance of himself, his family and the animal kingdom and the cleansing of the land we should, I believe, take his gesture to be one of thanksgiving and of dedication of the new heavens and the new earth to the glory of God:
Gen 8:21 CSB - 21 When the LORD smelled the pleasing aroma, he said to himself, "I will never again curse the ground because of human beings, even though the inclination of the human heart is evil from youth onward. And I will never again strike down every living thing as I have done.
The NT also refers to a "sacrifice of praise to God" both in words and in actions, which bring delight to God:
Heb 13:15-16 KJV - 15 By him therefore let us offer the sacrifice of praise to God continually, that is, the fruit of our lips giving thanks to his name. 16 But to do good and to communicate forget not: for with such sacrifices God is well pleased.
So to answer the question, Noah seems to have been praising and thanking God for his salvation and dedicating the new creation to his glory.
It may be that Peter's enigmatic comments comparing the flood to a giant mikveh allude to Noah's celebratory offering. Noah emerged from the flood as a type of the resurrection of Christ:
1Pe 3:21 NKJV - 21 There is also an antitype which now saves us--baptism (not the removal of the filth of the flesh, but the answer of a good conscience toward God), through the resurrection of Jesus Christ,
Noah sacrificed burnt offering after coming out of the ark. Gen 9:2 talks about the fear and dread of Abraham. Surely he must have been in awe of God for all that God had done. God accomplished everything just as he had promised. He must have have been overwhelmed by God's doing. He saw God from a new dimension. Imagine washing the earth out completely of all the sin and the sinners saving only Noah and his family. How BIG a God could have been so mindful of this simple Noah and his family too.