The ass is probably a decent indication for physical ability. The ability to generate power from the lower body? Not a lot of assless folks in professional sports, for example. So in olden times, when you needed to have a mate that was going to run down a deer and fight off the other tribe and make strong healthy babies, a nice ass was probably one of the traits you'd find in an ideal mate.
Probably a good indicator of health and genetics.
Probably a good indicator of health and genetics.