I've seen a lot of posts focusing on the visual symbolism of the scars, which is valid, but from a purely tactical standpoint, I think they force him into a corner that ends up defining his entire approach. Every time he goes all out, he's adding another permanent mark, another reminder of his physical limit. It's not just about power; it's a built-in countdown timer on his own body. So his fighting style evolves to be hyper-analytical, to find the one perfect point to strike without wasting a single percent of One For All. He can't brawl or trade blows like All Might used to; he has to be a precision instrument. The Manchester Smash against Lady Nagant is the ultimate example—calculating trajectory, wind resistance, everything, for a single, perfect shot that doesn't rely on sustained output. The scars are the cost of his power, and that cost forces him to fight smarter than anyone else on the field.
It also adds a brutal layer to the spectacle. When you see those scars flare up red and angry during a fight, you viscerally feel the strain. It makes every punch feel earned and dangerous, even when he wins. There's no clean victory for Deku; triumph is always etched into his skin afterwards.