>>13580227reason number one: fat storage and a certain fat distribution correlate with fertility. The fat around a woman's ass and thighs is used to support the fetus during pregnancy. It shows that she is healthy and in a position to bear a child.
reason number two: glutes attach to the hip bone, and ass that has more lateral voume shows that hips are wide enough to birth a child.
Perhaps the deeper question is why we take these ques from the buttox in the first place, which is succinctly answered in the fp.