Mouth rinses are classified into two categories by The Food and Drug Administration (FDA): therapeutic and cosmetic.
The therapeutic rinses that contain fluoride work to actually fight cavities, plaque and gingivitis.
Cosmetic rinses, however, only take care of bad breath, reduce any bacteria and get rid of food particles in the mouth. They will not treat or prevent gingivitis.
For those find it difficult to brush (due to physical infirmities like arthritis) can truly benefit from a recommended therapeutic mouth rinse.
Caution: Those rinses that state they can treat plaque or cavities, in reality, are only moderately effective. In fact, a regular practice of rinsing with water and using a quality fluoride toothpaste are equally or more effective.