Why does god need burnt offerings?

Gen 22:13 says, “And Abraham lifted up his eyes, and looked, and behold behind him a ram caught in a thicket by his horns: and Abraham went and took the ram, and offered him up for a burnt offering in the stead of his son.”  Why does god need burnt offerings?  He stops Abraham from murdering his son and then sets up a ram to be killed and burned.  What does this say about the Christian god?  Nowhere have I read why god wants things killed and burned for him.  The covenant was man chopping off his foreskin not killing and grilling.