Uniform cooking times.Large pieces of vegetables take longer to cook than smaller ones. So if you're sautéeing carrots that are cut to different sizes and shapes, you'll either overcook the smaller pieces by the time the bigger ones are done, or you'll cook the smaller pieces properly but leave the bigger ones undercooked. Consistent cutting technique ensures your food is cooked to a uniform degree of doneness.
Enhanced visual appeal.Again, it's the art part of the culinary arts. Of course, no one's going to take out a ruler and measure your knife cuts — unless you're in culinary school. But sloppy knife work makes for a sloppy-looking dish.
Skilled knife work indicates a cook who takes pride in their work and doesn't take shortcuts. It's a way of paying a compliment to whoever you're serving the dish to — saying to them, in effect, "You're worth the trouble."
Large Dice - Batonnet - Medium Dice
Allumette - Small Dice - Julienne
Brunoise - Fine Julienne - Fine Brunoise
Or to browse an image gallery showing all the basic knife cuts, see this Knife Cuts Photo Gallery.