They make the biggest difference for each individual player. But an experienced player will take a CDDA character (scenario where you start with a bad character, naked, sick and with bad wounds all over in a flaming building in the middle of winter) and somewhat easily make them into the ultimate survivor, whereas you can give a seriously op character to a newbie with all the skill already maxed out and no negative traits and they'll probably die to the first zombie encounter.

A lot of words to say that PZ is neither a numbers game nor really a mechanics game. Ultimately it's more of a knowledge game