system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why does the body store unnecessary amounts of fat, to the point where humans can become morbidly obese?
gpt
You have to keep in mind that access to enough food to eat yourself to death is a fairly modern thing to have.  Of the billions of years of life evolving that led to us, it's only in the last few thousand or so that this has been possible.

If you don't have access to this much food, "storing unnecessary amounts of fat" becomes "being able to eat more now and use it later."

Taking that into account, the ability to store dense reserves of energy (fat), which also happens to help keep you warm, is an extremely useful survival tool.  In fact, it's postulated that this is one of the evolutionary advances we have over other apes: a gorilla or chimp that massively overate like a person can would just die from it.  Being able to "stock up" internally is one of the abilities that allowed our ancestors to migrate through areas that didn't offer enough food.  The only other mammals that can put on arbitrary amounts of fat like we can and not die from it are aquatic (which also happens to be another situation where food can be scarce for long periods of time and staying warm is important).  Don't get me wrong: all mammals have fat stores, but humans can put on several times our normal weight in fat alone, and most mammals can't do this.