portrayal.
-it marks zoro's power limit before he unlocks power up. and speaks of his terrifying potential.
-it is an external attack, zoro is the only one in onigashima to do that, compared to the rest who aren't strong enough to do it instead they rely on tricks that bypass external defences like shockwaves/soundwaves/internal destruction.
-zoro inflicted it on kaido under an excruciating physical condition, a scar that kaido can't heal from and serves as a constant nerf for the rest of the fight, unlike the shallow damage that kaido can recover from instantly.
after zoro unlocked AdvCoC it's confirmed that he can slay kaido with one swing
one piece characters don't complain about wounds.