A dinner roll is small enough to set on a plate with your meat and potatoes. It usually includes milk and butter, as well as sugar, meaning that it’s soft, sweet, and tender. It can be buttered or not (it’s already buttery). It’s meant to be torn off and buttered, one piece at a time. Contrast that with, say, a loaf of crusty bread–it doesn’t include milk, butter, and sugar. It really DOES need to be spread with a little butter, and if it’s a bigger loaf, the slice might not fit as neatly on your plate. I’m not knocking crusty bread by any means here, but it’s no surprise that a dinner roll is a popular companion to a nice meal.