It's plumage.
Just like the male lyretails and peacocks have ridiculous tailfeathers- which DO impact ability to evade predators- to attract mates, 6-pack abs do attract a certain section of the human female populace.
It also turns some off. My Mom hates the ripped/muscular look. Good thing she married Dad- no chance of THAT!
Sure. Maybe I misphrased myself. I understand what it is. I was just observing the actual benefit level as compared to the cost, and that it seems to me that it really doesn't make all that much difference to one's life. I mean, I've had 'em, and not had 'em, and honestly my life was exactly the same except that some of the time I got to relax and eat nice stuff.