They are useful for flattening and for spanning from one bead to another on columns and such like.
They aren't any quicker, although you'll take less strokes you won't whip the trowel about as quickly as when using an 11in trowel.
IMO neither long or short trowels are best it's just personal choice, I'll stick to an 11in trowel for the vast majority of my work but there's always a 20in carbon steel trowel lurking for when I think it'd be advantageous. In fact used it a fair bit the last two days.