Unless you’re trying to get a passing grade in a culinary school knife skills class, or working for a demanding sous chef, you probably don’t need to double-check the exact measurements of a brunoise . But, the size and consistency of knife cuts really does matter; cutting vegetables uniformly helps ensure they cook at an even rate, and the size to which they are cut has a direct effect on their texture in a finished dish.
And, in the pastry kitchen, rulers really rule. When rolling out pie dough, for instance, the difference of an inch in diameter can mean a big difference in the final crust's thickness, and therefore, the success of your dessert: if it's too thin your bottom crust won't crisp up, and if it's too thick it can end up undercooked. The more intricate you get with your baking ambition, the more important accuracy becomes.