silly answer: to get to the other side.
more seriously, why would being yummy not be the point? isn;t that the whole point of of food?
If you want to know why we find it yummy, maybe this is what you're looking for: In the past and in many parts of the world, still today, when having enough calories was a major concern (unlike today, when having too many is the concern) any fat was good, and every culture had its preferred fat to put on bread or dip it into. But mainly, we like it because we are designed to accumulate calories so we don;t die of the starvation that was always around the corner, and still is in much of the world.