Dragging and dropping, as behaviors, go hand-in-hand with each other. Where one is found, the other is invariably close by. Dragging an element around a web page is all very well, but if there's nowhere for that element to be dragged to, then the whole exercise is usually pointless.
You can use the draggable class independently from the droppable class as pure dragging for the sake of dragging can have its uses, such as with the dialog component. However, you can't use the droppable class without the draggable. You don't need to make use of any of draggable's methods of course, but using droppables without having anything to drop onto them is of no value whatsoever.
Like with the widgets, it is possible however to combine some of the interaction helpers, draggables and droppables go together obviously. But draggables can also be used with sortables, as we'll see in Chapter 12.