If the purpose of putting pairs of animals in Noah's ark is to preserve the species then why did Noah offer them to God by burnt sacrifice when the water receded?

When Noah entered the ark he not only took the pairs for reproduction but also sevens (some debate exists whether this is seven animals or seven pairs) of the domesticated animals such as cattle. The idea here is that these tame animals would have been needed immediately after the flood for food. Also the suggestion is that this constitutes three breeding pairs and a seventh for sacrifice. Thus, the pairs of animals which were taken on the ark were a different and much larger group numerically than the domesticated group, even including the taking of sevens. The references for this are: Genesis 7:2-3 (2) "Of every clean beast thou shalt take to thee by sevens, the male and his female: and of beasts that are not clean by two, the male and his female. "(3) Of fowls also of the air by sevens, the male and the female; to keep seed alive upon the face of all the earth." Genesis 8:20 "And Noah built an altar unto the Lord; and took of every clean beast, and of every clean foul, and offered burnt offerings on the altar." These verses make it clear that there were animals for sacrifice apart from the 'unclean' or non-domesticated animals.god put sevens of things in the ark so i would imagine that the groups of sevens that noah took in to the ark he sacrificed some of them for god