Have you ever wondered why the buttocks is one of the most attractive parts of the body on both sexes? Here's the most common explanation I've heard mindlessly repeated: it's attractive because a man with a nice posterior will be better at thrusting during sex. I've also heard that it's purely aesthetic and nonfunctional, like a baboon's. Neither of these make any sense.
The shape of the buttocks comes mostly from the gluteal muscles (maximus and medius), superimposed by a layer of fat. The 'glutes' are some of the strongest muscles in the body, due to their large size and efficient leverage. Thrusting doesn't even come close to tapping into the glutes' tremendous power. What does? Heavy lifting. Sprints. Jumps. In short, some of the most functional full-body movements we perform as humans.
In any full-body movement, the hips are the central source of power. The strongest muscles surround the hips, and muscle strength diminishes progressively as you move further from them. A shapely buttocks is typically a strong buttocks, and a strong buttocks generally means a strong person. So if you want to decide at a glance whether a person is capable of sprinting and jumping after large prey, and then carrying it home, the buttocks is a good place to look.
The buttocks is also a storage area for fat. Humans tend to store a disproportionate amount of fat near their center of gravity: in the abdominal cavity, on the hips and on the buttocks. The right amount of fat indicates a healthy individual. A shapely buttocks is typically attached to someone who is strong and well-nourished. It's not so hard to imagine why we find it attractive.