It's easier to evenly heat a vertical slice of bread, for one. Most times I've used a toaster oven I've ended up with one side cooked much more than the other. Plus a horizontal toaster oven takes up a lot more counter/cabinet space, which is at a premium in a lot of kitchens. (I don't have a fancy toaster or fancy sandwich accessories, but I do really like the idea of multiple-use appliances.)