Horizontal punchers say the twist increases resistance to blocking. The twist on a punch resists attempts to deflect the punch. When a punch does not twist, all the forces in the punching arm are directed straight at the target. When a punch twists, the twisting forces are moving perpendicular to straight line to the target as they corkscrew down the arm to the target. It is relatively easy to deflect a punch that does not resist, other than with the inertia of its movement toward the target, so the non-twisting punch may be easily deflected. The twisting motion of the twisting punch resists any attempt to deflect the primary movement of the arm toward the target, which means a twisting punch is much more difficult to deflect than a non-twisting punch.
Compare a punch to the travel of bullet. When a bullet is fired down a smooth bore, it does not spin and is easily deflected by external forces, such as by the wind or a twig. When a bullet if fired down a rifled bore, the groves in the bore cause the bullet to twist along its axis toward the target. The spin helps the bullet resist any forces that may deflect it from its course to the target. Also, compare a punch to a thrown football. Without a spiral (spin) motion, the ball is unstable and easily deflected. With a spiral, the ball flies straighter, longer, and is not easily deflected.
Vertical punchers say that when the fist is held vertically, a block to the outside of the arm will be absorbed by both the radius and ulna bones, while in the horizontal punch, the radius bone takes the full impact. True. However, as discussed in other topics, the smaller the striking area, the greater the concentration of striking force. If the block strikes the wide top area of your forearm as it would in a vertical punch the less pain you may feel but also the less pain the blocker may feel. The whole point of your attack is to cause the opponent pain. If the block causes the blocker a lot of pain, whether the block stopped the attack or not, the next block by the opponent will probably be more tentative in an effort to ease the pain. As the attacker, if you are afraid of pain, then your attack will not be forceful and you will be tentative in your attacks. When you block with bone, it may hurt some but little damage is done and you can punch again. When you block with the muscles and tendons on the top of the forearm, and they are damaged, you may be unable to punch again.
In horizontal punch, the punching arm is protected against an inner or outer forearm block. In a vertical punch, the soft inner forearm with all its arteries, veins, muscles, ligaments, tendons, nerves, etc. is fully exposed. In a horizontal punch, your arm is protected no matter how the opponent blocks. In a vertical punch, the inner forearm is fully exposed. When fighting a vertical puncher, I would block their punches outward so I would strike their inner forearm, and I would even direct attacks to their inner forearms.