Uniform Knife Cuts Are About Even Cooking

Photo by John Harvey, Public domain, via Wikimedia Commons

Pieces of vegetable or meat cut to noticeably different sizes finish cooking at different times: the small pieces overcook and turn mushy while the large ones are still underdone, even though everything went into the pan at once. Cutting to a consistent size (whatever size the recipe calls for) means every piece hits the target texture at roughly the same moment — it's a functional habit that happens to also look tidier, not the other way around.