Sometimes you want to create a mood with your lighting. Dimmers not only add ambiance, but can help you create a number of different lighting schemes in one room.
For example, if you and your family are busy in the family room, but still want lights on in the kitchen, you can dim the lights in the latter room, turning them up to full strength when preparing a meal. Want to create a romantic setting in your bedroom? Use the dimmer controls to keep the lights on at a less intense level.