Even shoes marketed as 'wide' often are just wide in the middle of the shoe, and still taper at the toe. Virtually every shoe today has a narrow and not-foot-shaped toe box.
"But if every shoe has it, why does it matter? If it caused an issue, surely brands wouldn't have it"
Ahh, now you're on the right track of thinking. Unfortunately, shoes are seen as fashion with little regard for the actual foot that it goes on.
A wide toe box follows the shape of your foot, simply to allow your toes to sit in their normal anatomical position.
Ever seen a watermelon grown in a cubic square? It eventually grows to be shaped just like the thing its shoved into instead of it's natural round shape. The same thing happens to our feet if they are always shoved in a narrow and tapered toe box.
Wearing a shoe that allows the toes to splay is especially important for your gait and balance. Without toes that can spread out and stabilize, we have a much less stable base. Not only does this affect the foot's function, but the knee, hip, all the way up the chain. In addition to this, many years of wearing too tight and narrow of shoes can cause things like hammer toes, bunions, and neuromas.