Posture, meaning (in part) straight-spine or “elongated” spine position is a characteristic of hardstyle (Shuri-te) karate. I believe the point is to prevent back injury during the dangerous impact of full-power oi-zuki attacks, which I practice against a 150-lb body opponent bag. There is so much impact that the stress to the abdomen and back is overwhelming. I risk injury every time I try it. Only careful posture control and attention to focus prevents back injury.
However, the other two branches of karate... Nahate (goju, uechi, chuan fa) and the variations on Chan Migwa-te (Isshinryu and Matsubayashi Ryu)... do not place this emphasis on a straight backbone. They also don’t slam into their opponents with high-velocity body-mass impact. These styles stand still and chop/peck/poke/punch/gouge/grapple, and they use a flexible spine for greater suppleness and greater reach out of a largely stationary fighting posture.
One point that is often lost on Shotokan and hard-stylists... the straight backbone narrows our balance footprint, and makes us easy to knock over in some of our stances. A goju or isshinryu stylist may do his cat stance with his butt sticking out instead of tucked under... and he is much more resiliant to strikes and shoves in that posture than we are in ours.
This underlines a critical point that is generally misunderstood: basic principles exist for a reason, and are not universal. I think we misapply our Shotokan principles when we try to force them onto Chinese katas that never felt Matsumura’s hand.