Designers always seem to be trying to get the user's attention in as many unique and creative ways as possible. Clearly (GET IT?) the most common way is by visual reference. Layouts and control panels are designed and most uses and functions are learned by purely visual contact.
The second most common reference for control and affordable is sound. Sound is most strongly used for warnings and instant messages. Sound is a great aid in affordance because it can be sensed even when attention is elsewhere. This is effective with warnings primarily. Alarms and dings grab attention when it is most important...as long as they aren't used too often, then alls well.
Natural sounds (or sounds that make sense with functions) work most strongly when paired with other stimulus. For example, the iPod scroll where utilized the simple clicking signifying that the wheel is being turned.
This small addition to spinning a wheel makes the contact feel so much more tangible. For some reason, our mind connects the sound to the movement and the speed and volume the clicking helps aid to the scrolling of the wheel. It's a simple, but effective design note.